diff --git a/content/base/src/nsHostObjectProtocolHandler.cpp b/content/base/src/nsHostObjectProtocolHandler.cpp
index 7f6413149fc..d9aab865042 100644
--- a/content/base/src/nsHostObjectProtocolHandler.cpp
+++ b/content/base/src/nsHostObjectProtocolHandler.cpp
@@ -94,16 +94,14 @@ class BlobURLsReporter MOZ_FINAL : public nsIMemoryReporter
     }
 
     for (uint32_t i = 0; i < maxFrames && frame; ++i) {
-      nsAutoCString fileNameEscaped;
-      char* fileName = nullptr;
+      nsCString fileName;
       int32_t lineNumber = 0;
 
-      frame->GetFilename(&fileName);
+      frame->GetFilename(fileName);
       frame->GetLineNumber(&lineNumber);
 
-      if (fileName != nullptr && fileName[0] != '\0') {
+      if (!fileName.IsEmpty()) {
         stack += "js(";
-        fileNameEscaped = fileName;
         if (!origin.IsEmpty()) {
           // Make the file name root-relative for conciseness if possible.
           const char* originData;
@@ -111,14 +109,14 @@ class BlobURLsReporter MOZ_FINAL : public nsIMemoryReporter
 
           originLen = origin.GetData(&originData);
           // If fileName starts with origin + "/", cut up to that "/".
-          if (strlen(fileName) >= originLen + 1 &&
-              memcmp(fileName, originData, originLen) == 0 &&
+          if (fileName.Length() >= originLen + 1 &&
+              memcmp(fileName.get(), originData, originLen) == 0 &&
               fileName[originLen] == '/') {
-            fileNameEscaped.Cut(0, originLen);
+            fileName.Cut(0, originLen);
           }
         }
-        fileNameEscaped.ReplaceChar('/', '\\');
-        stack += fileNameEscaped;
+        fileName.ReplaceChar('/', '\\');
+        stack += fileName;
         if (lineNumber > 0) {
           stack += ", line=";
           stack.AppendInt(lineNumber);
diff --git a/dom/base/DOMError.cpp b/dom/base/DOMError.cpp
index 8da1b9ab791..2399075dac0 100644
--- a/dom/base/DOMError.cpp
+++ b/dom/base/DOMError.cpp
@@ -29,11 +29,11 @@ DOMError::DOMError(nsPIDOMWindow* aWindow)
 DOMError::DOMError(nsPIDOMWindow* aWindow, nsresult aValue)
   : mWindow(aWindow)
 {
-  const char *name, *message;
-  NS_GetNameAndMessageForDOMNSResult(aValue, &name, &message);
+  nsCString name, message;
+  NS_GetNameAndMessageForDOMNSResult(aValue, name, message);
 
-  mName = NS_ConvertASCIItoUTF16(name);
-  mMessage = NS_ConvertASCIItoUTF16(message);
+  CopyUTF8toUTF16(name, mName);
+  CopyUTF8toUTF16(message, mMessage);
 
   SetIsDOMBinding();
 }
diff --git a/dom/base/DOMException.cpp b/dom/base/DOMException.cpp
index b35bd6c41af..af5b6ed7b7d 100644
--- a/dom/base/DOMException.cpp
+++ b/dom/base/DOMException.cpp
@@ -89,17 +89,19 @@ static const struct ResultStruct
 
 static void
 NSResultToNameAndMessage(nsresult aNSResult,
-                         const char** aName,
-                         const char** aMessage,
+                         nsCString& aName,
+                         nsCString& aMessage,
                          uint16_t* aCode)
 {
-  *aName = nullptr;
-  *aMessage = nullptr;
+  aName.Truncate();
+  aMessage.Truncate();
   *aCode = 0;
   for (uint32_t idx = 0; idx < ArrayLength(sDOMErrorMsgMap); idx++) {
     if (aNSResult == sDOMErrorMsgMap[idx].mNSResult) {
-      *aName = sDOMErrorMsgMap[idx].mName;
-      *aMessage = sDOMErrorMsgMap[idx].mMessage;
+      aName.Rebind(sDOMErrorMsgMap[idx].mName,
+                   strlen(sDOMErrorMsgMap[idx].mName));
+      aMessage.Rebind(sDOMErrorMsgMap[idx].mMessage,
+                   strlen(sDOMErrorMsgMap[idx].mMessage));
       *aCode = sDOMErrorMsgMap[idx].mCode;
       return;
     }
@@ -111,17 +113,17 @@ NSResultToNameAndMessage(nsresult aNSResult,
 }
 
 nsresult
-NS_GetNameAndMessageForDOMNSResult(nsresult aNSResult, const char** aName,
-                                   const char** aMessage, uint16_t* aCode)
+NS_GetNameAndMessageForDOMNSResult(nsresult aNSResult, nsACString& aName,
+                                   nsACString& aMessage, uint16_t* aCode)
 {
-  const char* name = nullptr;
-  const char* message = nullptr;
+  nsCString name;
+  nsCString message;
   uint16_t code = 0;
-  NSResultToNameAndMessage(aNSResult, &name, &message, &code);
+  NSResultToNameAndMessage(aNSResult, name, message, &code);
 
-  if (name && message) {
-    *aName = name;
-    *aMessage = message;
+  if (!name.IsEmpty() && !message.IsEmpty()) {
+    aName = name;
+    aMessage = message;
     if (aCode) {
       *aCode = code;
     }
@@ -170,19 +172,13 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
 NS_IMPL_CI_INTERFACE_GETTER1(Exception, nsIXPCException)
 
-Exception::Exception(const char *aMessage,
+Exception::Exception(const nsACString& aMessage,
                      nsresult aResult,
-                     const char *aName,
+                     const nsACString& aName,
                      nsIStackFrame *aLocation,
                      nsISupports *aData)
-: mMessage(nullptr),
-  mResult(NS_OK),
-  mName(nullptr),
-  mLocation(nullptr),
-  mData(nullptr),
-  mFilename(nullptr),
+: mResult(NS_OK),
   mLineNumber(0),
-  mInner(nullptr),
   mInitialized(false),
   mHoldingJSVal(false)
 {
@@ -235,10 +231,7 @@ Exception::Exception(const char *aMessage,
 }
 
 Exception::Exception()
-  : mMessage(nullptr),
-    mResult(NS_OK),
-    mName(nullptr),
-    mFilename(nullptr),
+  : mResult(NS_OK),
     mLineNumber(-1),
     mInitialized(false),
     mHoldingJSVal(false)
@@ -247,19 +240,6 @@ Exception::Exception()
 
 Exception::~Exception()
 {
-  if (mMessage) {
-    nsMemory::Free(mMessage);
-    mMessage = nullptr;
-  }
-  if (mName) {
-    nsMemory::Free(mName);
-    mName = nullptr;
-  }
-  if (mFilename) {
-    nsMemory::Free(mFilename);
-    mFilename = nullptr;
-  }
-
   if (mHoldingJSVal) {
     MOZ_ASSERT(NS_IsMainThread());
 
@@ -296,20 +276,13 @@ Exception::StowJSVal(JS::Value& aVp)
   }
 }
 
-/* readonly attribute string message; */
+/* readonly attribute AUTF8String message; */
 NS_IMETHODIMP
-Exception::GetMessageMoz(char** aMessage)
+Exception::GetMessageMoz(nsACString& aMessage)
 {
-  NS_ENSURE_ARG_POINTER(aMessage);
   NS_ENSURE_TRUE(mInitialized, NS_ERROR_NOT_INITIALIZED);
 
-  if (mMessage) {
-    *aMessage =
-      (char*) nsMemory::Clone(mMessage, sizeof(char)*(strlen(mMessage)+1));
-  } else {
-    *aMessage = nullptr;
-  }
-
+  aMessage.Assign(mMessage);
   return NS_OK;
 }
 
@@ -324,30 +297,31 @@ Exception::GetResult(nsresult* aResult)
   return NS_OK;
 }
 
-/* readonly attribute string name; */
+/* readonly attribute AUTF8String name; */
 NS_IMETHODIMP
-Exception::GetName(char** aName)
+Exception::GetName(nsACString& aName)
 {
-  NS_ENSURE_ARG_POINTER(aName);
   NS_ENSURE_TRUE(mInitialized, NS_ERROR_NOT_INITIALIZED);
 
-  const char* name = mName;
-  if (!name) {
-    nsXPCException::NameAndFormatForNSResult(mResult, &name, nullptr);
-  }
-
-  if (name) {
-    *aName = (char*) nsMemory::Clone(name, sizeof(char)*(strlen(name)+1));
+  if (!mName.IsEmpty()) {
+    aName.Assign(mName);
   } else {
-    *aName = nullptr;
+    aName.Truncate();
+
+    const char* name = nullptr;
+    nsXPCException::NameAndFormatForNSResult(mResult, &name, nullptr);
+
+    if (name) {
+      aName.Assign(name);
+    }
   }
 
   return NS_OK;
 }
 
-/* readonly attribute string filename; */
+/* readonly attribute AUTF8String filename; */
 NS_IMETHODIMP
-Exception::GetFilename(char** aFilename)
+Exception::GetFilename(nsACString& aFilename)
 {
   NS_ENSURE_TRUE(mInitialized, NS_ERROR_NOT_INITIALIZED);
 
@@ -355,7 +329,8 @@ Exception::GetFilename(char** aFilename)
     return mLocation->GetFilename(aFilename);
   }
 
-  XPC_STRING_GETTER_BODY(aFilename, mFilename);
+  aFilename.Assign(mFilename);
+  return NS_OK;
 }
 
 /* readonly attribute uint32_t lineNumber; */
@@ -423,11 +398,10 @@ Exception::GetInner(nsIException** aException)
   return NS_OK;
 }
 
-/* string toString (); */
+/* AUTF8String toString (); */
 NS_IMETHODIMP
-Exception::ToString(char **_retval)
+Exception::ToString(nsACString& _retval)
 {
-  NS_ENSURE_ARG_POINTER(_retval);
   NS_ENSURE_TRUE(mInitialized, NS_ERROR_NOT_INITIALIZED);
 
   static const char defaultMsg[] = "<no message>";
@@ -435,18 +409,21 @@ Exception::ToString(char **_retval)
   static const char format[] =
 "[Exception... \"%s\"  nsresult: \"0x%x (%s)\"  location: \"%s\"  data: %s]";
 
-  char* indicatedLocation = nullptr;
+  nsCString location;
 
   if (mLocation) {
     // we need to free this if it does not fail
-    nsresult rv = mLocation->ToString(&indicatedLocation);
+    nsresult rv = mLocation->ToString(location);
     NS_ENSURE_SUCCESS(rv, rv);
   }
 
-  const char* msg = mMessage ? mMessage : nullptr;
-  const char* location = indicatedLocation ?
-                              indicatedLocation : defaultLocation;
-  const char* resultName = mName;
+  if (location.IsEmpty()) {
+    location.Assign(defaultLocation);
+  }
+
+  const char* msg = mMessage.IsEmpty() ? nullptr : mMessage.get();
+
+  const char* resultName = mName.IsEmpty() ? nullptr: mName.get();
   if (!resultName &&
       !nsXPCException::NameAndFormatForNSResult(mResult, &resultName,
                                                 (!msg) ? &msg : nullptr)) {
@@ -457,36 +434,24 @@ Exception::ToString(char **_retval)
   }
   const char* data = mData ? "yes" : "no";
 
-  char* temp = JS_smprintf(format, msg, mResult, resultName, location, data);
-  if (indicatedLocation) {
-    nsMemory::Free(indicatedLocation);
-  }
-
-  char* final = nullptr;
-  if (temp) {
-    final = (char*) nsMemory::Clone(temp, sizeof(char)*(strlen(temp)+1));
-    JS_smprintf_free(temp);
-  }
-
-  *_retval = final;
-  return final ? NS_OK : NS_ERROR_OUT_OF_MEMORY;
+  _retval.Truncate();
+  _retval.AppendPrintf(format, msg, mResult, resultName,
+                       location.get(), data);
+  return NS_OK;
 }
 
-/* void initialize (in string aMessage, in nsresult aResult, in string aName, in nsIStackFrame aLocation, in nsISupports aData, in nsIException aInner); */
+/* void initialize (in AUTF8String aMessage, in nsresult aResult,
+ *                  in AUTF8String aName, in nsIStackFrame aLocation,
+ *                  in nsISupports aData, in nsIException aInner); */
 NS_IMETHODIMP
-Exception::Initialize(const char *aMessage, nsresult aResult, const char *aName, nsIStackFrame *aLocation, nsISupports *aData, nsIException *aInner)
+Exception::Initialize(const nsACString& aMessage, nsresult aResult,
+                      const nsACString& aName, nsIStackFrame *aLocation,
+                      nsISupports *aData, nsIException *aInner)
 {
   NS_ENSURE_FALSE(mInitialized, NS_ERROR_ALREADY_INITIALIZED);
 
-  if (aMessage) {
-    mMessage =
-      (char*) nsMemory::Clone(aMessage, sizeof(char)*(strlen(aMessage)+1));
-  }
-
-  if (aName) {
-    mName = (char*) nsMemory::Clone(aName, sizeof(char)*(strlen(aName)+1));
-  }
-
+  mMessage = aMessage;
+  mName = aName;
   mResult = aResult;
 
   if (aLocation) {
@@ -516,14 +481,13 @@ Exception::WrapObject(JSContext* cx, JS::Handle<JSObject*> scope)
 void
 Exception::GetMessageMoz(nsString& retval)
 {
-  char* str = nullptr;
+  nsCString str;
 #ifdef DEBUG
   DebugOnly<nsresult> rv = 
 #endif
-  GetMessageMoz(&str);
+  GetMessageMoz(str);
   MOZ_ASSERT(NS_SUCCEEDED(rv));
   CopyUTF8toUTF16(str, retval);
-  nsMemory::Free(str);
 }
 
 uint32_t
@@ -535,27 +499,25 @@ Exception::Result() const
 void
 Exception::GetName(nsString& retval)
 {
-  char* str = nullptr;
+  nsCString str;
 #ifdef DEBUG
   DebugOnly<nsresult> rv =
 #endif
-  GetName(&str);
+  GetName(str);
   MOZ_ASSERT(NS_SUCCEEDED(rv));
   CopyUTF8toUTF16(str, retval);
-  nsMemory::Free(str);
 }
 
 void
 Exception::GetFilename(nsString& retval)
 {
-  char* str = nullptr;
+  nsCString str;
 #ifdef DEBUG
   DebugOnly<nsresult> rv =
 #endif
-  GetFilename(&str);
+  GetFilename(str);
   MOZ_ASSERT(NS_SUCCEEDED(rv));
   CopyUTF8toUTF16(str, retval);
-  nsMemory::Free(str);
 }
 
 uint32_t
@@ -602,14 +564,13 @@ Exception::GetData() const
 void
 Exception::Stringify(nsString& retval)
 {
-  char* str = nullptr;
+  nsCString str;
 #ifdef DEBUG
   DebugOnly<nsresult> rv =
 #endif
-  ToString(&str);
+  ToString(str);
   MOZ_ASSERT(NS_SUCCEEDED(rv));
   CopyUTF8toUTF16(str, retval);
-  nsMemory::Free(str);
 }
 
 NS_IMPL_ADDREF_INHERITED(DOMException, Exception)
@@ -618,9 +579,9 @@ NS_INTERFACE_MAP_BEGIN(DOMException)
   NS_INTERFACE_MAP_ENTRY(nsIDOMDOMException)
 NS_INTERFACE_MAP_END_INHERITING(Exception)
 
-DOMException::DOMException(nsresult aRv, const char* aMessage,
-                           const char* aName, uint16_t aCode)
-  : Exception(nullptr, aRv, nullptr, nullptr, nullptr),
+DOMException::DOMException(nsresult aRv, const nsACString& aMessage,
+                           const nsACString& aName, uint16_t aCode)
+  : Exception(EmptyCString(), aRv, EmptyCString(), nullptr, nullptr),
     mName(aName),
     mMessage(aMessage),
     mCode(aCode)
@@ -647,9 +608,9 @@ DOMException::GetCode(uint16_t* aCode)
 }
 
 NS_IMETHODIMP
-DOMException::ToString(char **aReturn)
+DOMException::ToString(nsACString& aReturn)
 {
-  *aReturn = nullptr;
+  aReturn.Truncate();
 
   static const char defaultMsg[] = "<no message>";
   static const char defaultLocation[] = "<unknown>";
@@ -660,9 +621,8 @@ DOMException::ToString(char **aReturn)
   nsAutoCString location;
 
   if (mInner) {
-    nsXPIDLCString filename;
-
-    mInner->GetFilename(getter_Copies(filename));
+    nsCString filename;
+    mInner->GetFilename(filename);
 
     if (!filename.IsEmpty()) {
       uint32_t line_nr = 0;
@@ -681,13 +641,13 @@ DOMException::ToString(char **aReturn)
     location = defaultLocation;
   }
 
-  const char* msg = mMessage ? mMessage : defaultMsg;
-  const char* resultName = mName ? mName : defaultName;
+  const char* msg = !mMessage.IsEmpty() ? mMessage.get() : defaultMsg;
+  const char* resultName = !mName.IsEmpty() ? mName.get() : defaultName;
 
-  *aReturn = PR_smprintf(format, msg, mCode, mResult, resultName,
-                         location.get());
+  aReturn.AppendPrintf(format, msg, mCode, mResult, resultName,
+                       location.get());
 
-  return *aReturn ? NS_OK : NS_ERROR_OUT_OF_MEMORY;
+  return NS_OK;
 }
 
 void
@@ -711,10 +671,10 @@ DOMException::WrapObject(JSContext* aCx, JS::Handle<JSObject*> aScope)
 /* static */already_AddRefed<DOMException>
 DOMException::Create(nsresult aRv)
 {
-  const char* name;
-  const char* message;
+  nsCString name;
+  nsCString message;
   uint16_t code;
-  NSResultToNameAndMessage(aRv, &name, &message, &code);
+  NSResultToNameAndMessage(aRv, name, message, &code);
   nsRefPtr<DOMException> inst =
     new DOMException(aRv, message, name, code);
   return inst.forget();
diff --git a/dom/base/DOMException.h b/dom/base/DOMException.h
index cdf5ebeaeac..58744ac771f 100644
--- a/dom/base/DOMException.h
+++ b/dom/base/DOMException.h
@@ -21,13 +21,14 @@
 #include "nsIDOMDOMException.h"
 #include "nsWrapperCache.h"
 #include "xpcexception.h"
+#include "nsString.h"
 
 class nsIStackFrame;
 class nsString;
 
 nsresult
-NS_GetNameAndMessageForDOMNSResult(nsresult aNSResult, const char** aName,
-                                   const char** aMessage,
+NS_GetNameAndMessageForDOMNSResult(nsresult aNSResult, nsACString& aName,
+                                   nsACString& aMessage,
                                    uint16_t* aCode = nullptr);
 
 namespace mozilla {
@@ -85,21 +86,21 @@ public:
   // XPCOM factory ctor.
   Exception();
 
-  Exception(const char *aMessage,
+  Exception(const nsACString& aMessage,
             nsresult aResult,
-            const char *aName,
+            const nsACString& aName,
             nsIStackFrame *aLocation,
             nsISupports *aData);
 
 protected:
   virtual ~Exception();
 
-  char*           mMessage;
+  nsCString       mMessage;
   nsresult        mResult;
-  char*           mName;
+  nsCString       mName;
   nsCOMPtr<nsIStackFrame> mLocation;
   nsCOMPtr<nsISupports> mData;
-  char*           mFilename;
+  nsCString       mFilename;
   int             mLineNumber;
   nsCOMPtr<nsIException> mInner;
   bool            mInitialized;
@@ -117,14 +118,14 @@ class DOMException : public Exception,
                      public nsIDOMDOMException
 {
 public:
-  DOMException(nsresult aRv, const char* aMessage,
-               const char* aName, uint16_t aCode);
+  DOMException(nsresult aRv, const nsACString& aMessage,
+               const nsACString& aName, uint16_t aCode);
 
   NS_DECL_ISUPPORTS_INHERITED
   NS_DECL_NSIDOMDOMEXCEPTION
 
   // nsIException overrides
-  NS_IMETHOD ToString(char **aReturn) MOZ_OVERRIDE;
+  NS_IMETHOD ToString(nsACString& aReturn) MOZ_OVERRIDE;
 
   // nsWrapperCache overrides
   virtual JSObject* WrapObject(JSContext* aCx, JS::Handle<JSObject*> aScope)
@@ -145,9 +146,8 @@ protected:
 
   virtual ~DOMException() {}
 
-  // Intentionally shadow the nsXPCException version.
-  const char* mName;
-  const char* mMessage;
+  nsCString mName;
+  nsCString mMessage;
 
   uint16_t mCode;
 };
diff --git a/dom/bindings/Exceptions.cpp b/dom/bindings/Exceptions.cpp
index 7c169dcb3a8..d4083d50e1c 100644
--- a/dom/bindings/Exceptions.cpp
+++ b/dom/bindings/Exceptions.cpp
@@ -147,7 +147,8 @@ Throw(JSContext* aCx, nsresult aRv, const char* aMessage)
 
   // If not, use the default.
   if (!finalException) {
-    finalException = new Exception(aMessage, aRv, nullptr, nullptr, nullptr);
+    finalException = new Exception(nsCString(aMessage), aRv,
+                                   EmptyCString(), nullptr, nullptr);
   }
 
   MOZ_ASSERT(finalException);
@@ -292,16 +293,14 @@ private:
     return mLanguage == nsIProgrammingLanguage::JAVASCRIPT;
   }
 
-  const char* GetFilename();
-  const char* GetFunname();
   int32_t GetLineno();
 
   nsRefPtr<StackDescriptionOwner> mStackDescription;
   nsCOMPtr<nsIStackFrame> mCaller;
 
   // Cached values
-  char* mFilename;
-  char* mFunname;
+  nsCString mFilename;
+  nsCString mFunname;
   int32_t mLineno;
   uint32_t mLanguage;
 
@@ -315,9 +314,7 @@ private:
 
 JSStackFrame::JSStackFrame(StackDescriptionOwner* aStackDescription,
                            size_t aIndex)
-  : mFilename(nullptr),
-    mFunname(nullptr),
-    mLineno(0)
+  : mLineno(0)
 {
   if (aStackDescription && aIndex < aStackDescription->NumFrames()) {
     mStackDescription = aStackDescription;
@@ -340,12 +337,6 @@ JSStackFrame::JSStackFrame(StackDescriptionOwner* aStackDescription,
 
 JSStackFrame::~JSStackFrame()
 {
-  if (mFilename) {
-    nsMemory::Free(mFilename);
-  }
-  if (mFunname) {
-    nsMemory::Free(mFunname);
-  }
 }
 
 NS_IMPL_CYCLE_COLLECTION_2(JSStackFrame, mStackDescription, mCaller)
@@ -366,22 +357,22 @@ NS_IMETHODIMP JSStackFrame::GetLanguage(uint32_t* aLanguage)
 }
 
 /* readonly attribute string languageName; */
-NS_IMETHODIMP JSStackFrame::GetLanguageName(char** aLanguageName)
+NS_IMETHODIMP JSStackFrame::GetLanguageName(nsACString& aLanguageName)
 {
   static const char js[] = "JavaScript";
   static const char cpp[] = "C++";
 
   if (IsJSFrame()) {
-    *aLanguageName = (char*) nsMemory::Clone(js, sizeof(js));
+    aLanguageName.AssignASCII(js);
   } else {
-    *aLanguageName = (char*) nsMemory::Clone(cpp, sizeof(cpp));
+    aLanguageName.AssignASCII(cpp);
   }
 
   return NS_OK;
 }
 
-const char*
-JSStackFrame::GetFilename()
+/* readonly attribute string filename; */
+NS_IMETHODIMP JSStackFrame::GetFilename(nsACString& aFilename)
 {
   if (!mFilenameInitialized) {
     JS::FrameDescription& desc = mStackDescription->FrameAt(mIndex);
@@ -393,34 +384,22 @@ JSStackFrame::GetFilename()
       JSAutoCompartment ac(cx, desc.script());
       const char* filename = JS_GetScriptFilename(cx, desc.script());
       if (filename) {
-        mFilename =
-          (char*)nsMemory::Clone(filename, sizeof(char)*(strlen(filename)+1));
+        mFilename.Assign(filename);
       }
     }
     mFilenameInitialized = true;
   }
 
-  return mFilename;
-}
-
-/* readonly attribute string filename; */
-NS_IMETHODIMP JSStackFrame::GetFilename(char** aFilename)
-{
-  NS_ENSURE_ARG_POINTER(aFilename);
-
-  const char* filename = GetFilename();
-  if (filename) {
-    *aFilename = (char*) nsMemory::Clone(filename,
-                                         sizeof(char)*(strlen(filename)+1));
+  if (mFilename.IsEmpty()) {
+    aFilename.SetIsVoid(true);
   } else {
-    *aFilename = nullptr;
+    aFilename.Assign(mFilename);
   }
-
   return NS_OK;
 }
 
-const char*
-JSStackFrame::GetFunname()
+/* readonly attribute string name; */
+NS_IMETHODIMP JSStackFrame::GetName(nsACString& aFunction)
 {
   if (!mFunnameInitialized) {
     JS::FrameDescription& desc = mStackDescription->FrameAt(mIndex);
@@ -433,33 +412,19 @@ JSStackFrame::GetFunname()
       if (funid) {
         size_t length = JS_GetStringEncodingLength(cx, funid);
         if (length != size_t(-1)) {
-          mFunname = static_cast<char *>(nsMemory::Alloc(length + 1));
-          if (mFunname) {
-            JS_EncodeStringToBuffer(cx, funid, mFunname, length);
-            mFunname[length] = '\0';
-          }
+          mFunname.SetLength(uint32_t(length));
+          JS_EncodeStringToBuffer(cx, funid, mFunname.BeginWriting(), length);
         }
       }
     }
     mFunnameInitialized = true;
   }
 
-  return mFunname;
-}
-
-/* readonly attribute string name; */
-NS_IMETHODIMP JSStackFrame::GetName(char** aFunction)
-{
-  NS_ENSURE_ARG_POINTER(aFunction);
-
-  const char* funname = GetFunname();
-  if (funname) {
-    *aFunction = (char*) nsMemory::Clone(funname,
-                                         sizeof(char)*(strlen(funname)+1));
+  if (mFunname.IsEmpty()) {
+    aFunction.SetIsVoid(true);
   } else {
-    *aFunction = nullptr;
+    aFunction.Assign(mFunname);
   }
-
   return NS_OK;
 }
 
@@ -482,10 +447,10 @@ NS_IMETHODIMP JSStackFrame::GetLineNumber(int32_t* aLineNumber)
   return NS_OK;
 }
 
-/* readonly attribute string sourceLine; */
-NS_IMETHODIMP JSStackFrame::GetSourceLine(char** aSourceLine)
+/* readonly attribute AUTF8String sourceLine; */
+NS_IMETHODIMP JSStackFrame::GetSourceLine(nsACString& aSourceLine)
 {
-  *aSourceLine = nullptr;
+  aSourceLine.Truncate();
   return NS_OK;
 }
 
@@ -500,26 +465,31 @@ NS_IMETHODIMP JSStackFrame::GetCaller(nsIStackFrame** aCaller)
   return NS_OK;
 }
 
-/* string toString (); */
-NS_IMETHODIMP JSStackFrame::ToString(char** _retval)
+/* AUTF8String toString (); */
+NS_IMETHODIMP JSStackFrame::ToString(nsACString& _retval)
 {
+  _retval.Truncate();
+
   const char* frametype = IsJSFrame() ? "JS" : "native";
-  const char* filename = GetFilename();
-  if (!filename) {
-    filename = "<unknown filename>";
+
+  nsCString filename;
+  nsresult rv = GetFilename(filename);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (filename.IsEmpty()) {
+    filename.AssignASCII("<unknown filename>");
   }
-  const char* funname = GetFunname();
-  if (!funname) {
-    funname = "<TOP_LEVEL>";
+
+  nsCString funname;
+  rv = GetName(funname);
+  NS_ENSURE_SUCCESS(rv, rv);
+
+  if (funname.IsEmpty()) {
+    funname.AssignASCII("<TOP_LEVEL>");
   }
   static const char format[] = "%s frame :: %s :: %s :: line %d";
-  int len = sizeof(char)*
-              (strlen(frametype) + strlen(filename) + strlen(funname)) +
-            sizeof(format) + 3 * sizeof(mLineno);
-
-  char* buf = (char*) nsMemory::Alloc(len);
-  JS_snprintf(buf, len, format, frametype, filename, funname, GetLineno());
-  *_retval = buf;
+  _retval.AppendPrintf(format, frametype, filename.get(),
+                       funname.get(), GetLineno());
   return NS_OK;
 }
 
@@ -550,17 +520,8 @@ JSStackFrame::CreateStackFrameLocation(uint32_t aLanguage,
 
   self->mLanguage = aLanguage;
   self->mLineno = aLineNumber;
-
-  if (aFilename) {
-    self->mFilename =
-      (char*)nsMemory::Clone(aFilename, sizeof(char)*(strlen(aFilename)+1));
-  }
-
-  if (aFunctionName) {
-    self->mFunname = 
-      (char*)nsMemory::Clone(aFunctionName,
-                             sizeof(char)*(strlen(aFunctionName)+1));
-  }
+  self->mFilename = aFilename;
+  self->mFunname = aFunctionName;
 
   self->mCaller = aCaller;
 
diff --git a/js/xpconnect/idl/xpccomponents.idl b/js/xpconnect/idl/xpccomponents.idl
index 6f39ceed02e..0488db37cf8 100644
--- a/js/xpconnect/idl/xpccomponents.idl
+++ b/js/xpconnect/idl/xpccomponents.idl
@@ -153,7 +153,7 @@ interface nsIXPCComponents_Utils : nsISupports
     [implicit_jscontext,optional_argc]
     jsval evalInSandbox(in AString source, in jsval sandbox,
                         [optional] in jsval version,
-                        [optional] in jsval filename,
+                        [optional] in AUTF8String filename,
                         [optional] in long lineNo);
 
     /*
diff --git a/js/xpconnect/idl/xpcexception.idl b/js/xpconnect/idl/xpcexception.idl
index 79db29aced9..026766a4c88 100644
--- a/js/xpconnect/idl/xpcexception.idl
+++ b/js/xpconnect/idl/xpcexception.idl
@@ -12,9 +12,9 @@ interface nsIXPCException : nsIException
 {
     // inherits methods from nsIException
 
-    void initialize(in string           aMessage,
+    void initialize(in AUTF8String      aMessage,
                     in nsresult         aResult,
-                    in string           aName,
+                    in AUTF8String      aName,
                     in nsIStackFrame    aLocation,
                     in nsISupports      aData,
                     in nsIException     aInner);
diff --git a/js/xpconnect/src/Sandbox.cpp b/js/xpconnect/src/Sandbox.cpp
index 9b7a7fbb966..7404415cbd3 100644
--- a/js/xpconnect/src/Sandbox.cpp
+++ b/js/xpconnect/src/Sandbox.cpp
@@ -1515,7 +1515,7 @@ AssembleSandboxMemoryReporterName(JSContext *cx, nsCString &sandboxName)
     if (frame) {
         nsCString location;
         int32_t lineNumber = 0;
-        frame->GetFilename(getter_Copies(location));
+        frame->GetFilename(location);
         frame->GetLineNumber(&lineNumber);
 
         sandboxName.AppendLiteral(" (from: ");
@@ -1634,7 +1634,7 @@ ContextHolder::~ContextHolder()
 
 nsresult
 xpc::EvalInSandbox(JSContext *cx, HandleObject sandboxArg, const nsAString& source,
-                   const char *filename, int32_t lineNo,
+                   const nsACString& filename, int32_t lineNo,
                    JSVersion jsVersion, bool returnStringOnly, MutableHandleValue rval)
 {
     JS_AbortIfWrongThread(JS_GetRuntime(cx));
@@ -1653,10 +1653,11 @@ xpc::EvalInSandbox(JSContext *cx, HandleObject sandboxArg, const nsAString& sour
     NS_ENSURE_TRUE(prin, NS_ERROR_FAILURE);
 
     nsAutoCString filenameBuf;
-    if (!filename) {
+    if (!filename.IsVoid()) {
+        filenameBuf.Assign(filename);
+    } else {
         // Default to the spec of the principal.
         nsJSPrincipals::get(prin)->GetScriptLocation(filenameBuf);
-        filename = filenameBuf.get();
         lineNo = 1;
     }
 
@@ -1680,7 +1681,7 @@ xpc::EvalInSandbox(JSContext *cx, HandleObject sandboxArg, const nsAString& sour
 
         JS::CompileOptions options(sandcx);
         options.setPrincipals(nsJSPrincipals::get(prin))
-               .setFileAndLine(filename, lineNo);
+               .setFileAndLine(filenameBuf.get(), lineNo);
         if (jsVersion != JSVERSION_DEFAULT)
                options.setVersion(jsVersion);
         JS::RootedObject rootedSandbox(sandcx, sandbox);
diff --git a/js/xpconnect/src/XPCComponents.cpp b/js/xpconnect/src/XPCComponents.cpp
index efb6a50f548..c0148208c90 100644
--- a/js/xpconnect/src/XPCComponents.cpp
+++ b/js/xpconnect/src/XPCComponents.cpp
@@ -1861,8 +1861,10 @@ nsXPCComponents_Exception::CallOrConstruct(nsIXPConnectWrappedNative *wrapper,
     if (!parser.parse(args))
         return ThrowAndFail(NS_ERROR_XPC_BAD_CONVERT_JS, cx, _retval);
 
-    nsCOMPtr<nsIException> e = new Exception(parser.eMsg, parser.eResult,
-                                             nullptr, parser.eStack,
+    nsCOMPtr<nsIException> e = new Exception(nsCString(parser.eMsg),
+                                             parser.eResult,
+                                             EmptyCString(),
+                                             parser.eStack,
                                              parser.eData);
 
     nsCOMPtr<nsIXPConnectJSObjectHolder> holder;
@@ -2582,10 +2584,10 @@ nsXPCComponents_Utils::ReportError(HandleValue error, JSContext *cx)
     nsXPConnect *xpc = nsXPConnect::XPConnect();
     xpc->GetCurrentJSStack(getter_AddRefs(frame));
 
-    nsXPIDLCString fileName;
+    nsCString fileName;
     int32_t lineNo = 0;
     if (frame) {
-        frame->GetFilename(getter_Copies(fileName));
+        frame->GetFilename(fileName);
         frame->GetLineNumber(&lineNo);
     }
 
@@ -2595,8 +2597,8 @@ nsXPCComponents_Utils::ReportError(HandleValue error, JSContext *cx)
 
     nsresult rv = scripterr->InitWithWindowID(
             nsDependentString(static_cast<const char16_t *>(msgchars)),
-            NS_ConvertUTF8toUTF16(fileName),
-            EmptyString(), lineNo, 0, 0, "XPConnect JavaScript", innerWindowID);
+            NS_ConvertUTF8toUTF16(fileName), EmptyString(), lineNo, 0, 0,
+            "XPConnect JavaScript", innerWindowID);
     NS_ENSURE_SUCCESS(rv, NS_OK);
 
     console->LogMessage(scripterr);
@@ -2608,7 +2610,7 @@ NS_IMETHODIMP
 nsXPCComponents_Utils::EvalInSandbox(const nsAString& source,
                                      HandleValue sandboxVal,
                                      HandleValue version,
-                                     HandleValue filenameVal,
+                                     const nsACString& filenameArg,
                                      int32_t lineNumber,
                                      JSContext *cx,
                                      uint8_t optionalArgc,
@@ -2642,17 +2644,10 @@ nsXPCComponents_Utils::EvalInSandbox(const nsAString& source,
     }
 
     // Optional fourth and fifth arguments: filename and line number.
-    nsXPIDLCString filename;
     int32_t lineNo = (optionalArgc >= 3) ? lineNumber : 1;
-    if (optionalArgc >= 2) {
-        JSString *filenameStr = ToString(cx, filenameVal);
-        if (!filenameStr)
-            return NS_ERROR_INVALID_ARG;
-
-        JSAutoByteString filenameBytes;
-        if (!filenameBytes.encodeLatin1(cx, filenameStr))
-            return NS_ERROR_INVALID_ARG;
-        filename = filenameBytes.ptr();
+    nsCString filename;
+    if (!filenameArg.IsVoid()) {
+        filename.Assign(filenameArg);
     } else {
         // Get the current source info from xpc.
         nsresult rv;
@@ -2662,12 +2657,12 @@ nsXPCComponents_Utils::EvalInSandbox(const nsAString& source,
         nsCOMPtr<nsIStackFrame> frame;
         xpc->GetCurrentJSStack(getter_AddRefs(frame));
         if (frame) {
-            frame->GetFilename(getter_Copies(filename));
+            frame->GetFilename(filename);
             frame->GetLineNumber(&lineNo);
         }
     }
 
-    return xpc::EvalInSandbox(cx, sandbox, source, filename.get(), lineNo,
+    return xpc::EvalInSandbox(cx, sandbox, source, filename, lineNo,
                               jsVersion, false, retval);
 }
 
diff --git a/js/xpconnect/src/XPCConvert.cpp b/js/xpconnect/src/XPCConvert.cpp
index c8b6c294e30..ea97c37e4f5 100644
--- a/js/xpconnect/src/XPCConvert.cpp
+++ b/js/xpconnect/src/XPCConvert.cpp
@@ -1070,7 +1070,6 @@ XPCConvert::ConstructException(nsresult rv, const char* message,
 
     static const char format[] = "\'%s\' when calling method: [%s::%s]";
     const char * msg = message;
-    char* sz = nullptr;
     nsXPIDLString xmsg;
     nsAutoCString sxmsg;
 
@@ -1084,19 +1083,18 @@ XPCConvert::ConstructException(nsresult rv, const char* message,
     if (!msg)
         if (!nsXPCException::NameAndFormatForNSResult(rv, nullptr, &msg) || ! msg)
             msg = "<error>";
-    if (ifaceName && methodName)
-        msg = sz = JS_smprintf(format, msg, ifaceName, methodName);
 
-    nsRefPtr<Exception> e = new Exception(msg, rv, nullptr, nullptr, data);
+    nsCString msgStr(msg);
+    if (ifaceName && methodName)
+        msgStr.AppendPrintf(format, msg, ifaceName, methodName);
+
+    nsRefPtr<Exception> e = new Exception(msgStr, rv, EmptyCString(), nullptr, data);
 
     if (cx && jsExceptionPtr) {
         e->StowJSVal(*jsExceptionPtr);
     }
 
     e.forget(exceptn);
-
-    if (sz)
-        JS_smprintf_free(sz);
     return NS_OK;
 }
 
diff --git a/js/xpconnect/src/XPCWrappedJSClass.cpp b/js/xpconnect/src/XPCWrappedJSClass.cpp
index 2cd7150cd3f..a336fb2244a 100644
--- a/js/xpconnect/src/XPCWrappedJSClass.cpp
+++ b/js/xpconnect/src/XPCWrappedJSClass.cpp
@@ -899,11 +899,11 @@ nsXPCWrappedJSClass::CheckForException(XPCCallContext & ccx,
 
                     fputs(line, stdout);
                     fputs(preamble, stdout);
-                    char* text;
-                    if (NS_SUCCEEDED(xpc_exception->ToString(&text)) && text) {
-                        fputs(text, stdout);
+                    nsCString text;
+                    if (NS_SUCCEEDED(xpc_exception->ToString(text)) &&
+                        !text.IsEmpty()) {
+                        fputs(text.get(), stdout);
                         fputs("\n", stdout);
-                        nsMemory::Free(text);
                     } else
                         fputs(cant_get_text, stdout);
                     fputs(line, stdout);
@@ -926,17 +926,13 @@ nsXPCWrappedJSClass::CheckForException(XPCCallContext & ccx,
                         // try to cook one up.
                         scriptError = do_CreateInstance(XPC_SCRIPT_ERROR_CONTRACTID);
                         if (nullptr != scriptError) {
-                            char* exn_string;
-                            rv = xpc_exception->ToString(&exn_string);
+                            nsCString newMessage;
+                            rv = xpc_exception->ToString(newMessage);
                             if (NS_SUCCEEDED(rv)) {
-                                // use toString on the exception as the message
-                                NS_ConvertASCIItoUTF16 newMessage(exn_string);
-                                nsMemory::Free((void *) exn_string);
-
                                 // try to get filename, lineno from the first
                                 // stack frame location.
                                 int32_t lineNumber = 0;
-                                nsXPIDLCString sourceName;
+                                nsCString sourceName;
 
                                 nsCOMPtr<nsIStackFrame> location;
                                 xpc_exception->
@@ -946,11 +942,11 @@ nsXPCWrappedJSClass::CheckForException(XPCCallContext & ccx,
                                     location->GetLineNumber(&lineNumber);
 
                                     // get a filename.
-                                    rv = location->GetFilename(getter_Copies(sourceName));
+                                    rv = location->GetFilename(sourceName);
                                 }
 
-                                rv = scriptError->InitWithWindowID(newMessage,
-                                                                   NS_ConvertASCIItoUTF16(sourceName),
+                                rv = scriptError->InitWithWindowID(NS_ConvertUTF8toUTF16(newMessage),
+                                                                   NS_ConvertUTF8toUTF16(sourceName),
                                                                    EmptyString(),
                                                                    lineNumber, 0, 0,
                                                                    "XPConnect JavaScript",
diff --git a/js/xpconnect/src/nsXPConnect.cpp b/js/xpconnect/src/nsXPConnect.cpp
index ad911f19a3f..19137da2caa 100644
--- a/js/xpconnect/src/nsXPConnect.cpp
+++ b/js/xpconnect/src/nsXPConnect.cpp
@@ -898,9 +898,14 @@ nsXPConnect::EvalInSandboxObject(const nsAString& source, const char *filename,
         return NS_ERROR_INVALID_ARG;
 
     RootedObject sandbox(cx, sandboxArg);
-    return EvalInSandbox(cx, sandbox, source, filename ? filename :
-                         "x-bogus://XPConnect/Sandbox", 1, JSVERSION_DEFAULT,
-                         returnStringOnly, rval);
+    nsCString filenameStr;
+    if (filename) {
+        filenameStr.Assign(filename);
+    } else {
+        filenameStr = NS_LITERAL_CSTRING("x-bogus://XPConnect/Sandbox");
+    }
+    return EvalInSandbox(cx, sandbox, source, filenameStr, 1,
+                         JSVERSION_DEFAULT, returnStringOnly, rval);
 }
 
 /* nsIXPConnectJSObjectHolder getWrappedNativePrototype (in JSContextPtr aJSContext, in JSObjectPtr aScope, in nsIClassInfo aClassInfo); */
diff --git a/js/xpconnect/src/xpcprivate.h b/js/xpconnect/src/xpcprivate.h
index 4021f49a75c..fc70554855d 100644
--- a/js/xpconnect/src/xpcprivate.h
+++ b/js/xpconnect/src/xpcprivate.h
@@ -3527,7 +3527,7 @@ CreateSandboxObject(JSContext *cx, JS::MutableHandleValue vp, nsISupports *prinO
 // result, and cx->exception will be empty.
 nsresult
 EvalInSandbox(JSContext *cx, JS::HandleObject sandbox, const nsAString& source,
-              const char *filename, int32_t lineNo,
+              const nsACString& filename, int32_t lineNo,
               JSVersion jsVersion, bool returnStringOnly,
               JS::MutableHandleValue rval);
 
diff --git a/xpcom/base/nsIException.idl b/xpcom/base/nsIException.idl
index 77b760f4713..ef7a808cb07 100644
--- a/xpcom/base/nsIException.idl
+++ b/xpcom/base/nsIException.idl
@@ -10,35 +10,31 @@
 
 #include "nsISupports.idl"
 
-// XXX - most "string"s in this file should probably move to Unicode
-//       so may as well use AStrings...
-
-
 [scriptable, uuid(91d82105-7c62-4f8b-9779-154277c0ee90)]
 interface nsIStackFrame : nsISupports
 {
     // see nsIProgrammingLanguage for list of language consts
     readonly attribute uint32_t                language;
-    readonly attribute string                  languageName;
-    readonly attribute string                  filename;
-    readonly attribute string                  name;
+    readonly attribute AUTF8String             languageName;
+    readonly attribute AUTF8String             filename;
+    readonly attribute AUTF8String             name;
     // Valid line numbers begin at '1'. '0' indicates unknown.
     readonly attribute int32_t                 lineNumber;
-    readonly attribute string                  sourceLine;
+    readonly attribute AUTF8String             sourceLine;
     readonly attribute nsIStackFrame           caller;
 
-    string toString();
+    AUTF8String toString();
 };
 
 [scriptable, uuid(F3A8D3B4-C424-4edc-8BF6-8974C983BA78)]
 interface nsIException : nsISupports
 {
     // A custom message set by the thrower.
-    [binaryname(MessageMoz)] readonly attribute string message;
+    [binaryname(MessageMoz)] readonly attribute AUTF8String message;
     // The nsresult associated with this exception.
     readonly attribute nsresult                result;
     // The name of the error code (ie, a string repr of |result|)
-    readonly attribute string                  name;
+    readonly attribute AUTF8String             name;
 
     // Filename location.  This is the location that caused the
     // error, which may or may not be a source file location.
@@ -48,7 +44,7 @@ interface nsIException : nsISupports
     // etc.
 
     // null indicates "no data"
-    readonly attribute string                  filename;
+    readonly attribute AUTF8String             filename;
     // Valid line numbers begin at '1'. '0' indicates unknown.
     readonly attribute uint32_t                lineNumber;
     // Valid column numbers begin at 0. 
@@ -64,5 +60,5 @@ interface nsIException : nsISupports
     readonly attribute nsISupports             data;
 
     // A generic formatter - make it suitable to print, etc.
-    string toString();
+    AUTF8String toString();
 };