diff --git a/image/src/RasterImage.cpp b/image/src/RasterImage.cpp
index ee9f6989f61..6a8e3d38383 100644
--- a/image/src/RasterImage.cpp
+++ b/image/src/RasterImage.cpp
@@ -1531,7 +1531,7 @@ RasterImage::AddSourceData(const char *aBuffer, PRUint32 aCount)
 /* Note!  buf must be declared as char buf[9]; */
 // just used for logging and hashing the header
 static void
-get_header_str (char *buf, char *data, PRSize data_len)
+get_header_str (char *buf, char *data, size_t data_len)
 {
   int i;
   int n;
diff --git a/layout/base/nsBidi.cpp b/layout/base/nsBidi.cpp
index 0a797bfdf63..86eadbfb7d0 100644
--- a/layout/base/nsBidi.cpp
+++ b/layout/base/nsBidi.cpp
@@ -184,7 +184,7 @@ void nsBidi::Init()
  * which we know we don't need any more;
  * is this the best way to do this??
  */
-bool nsBidi::GetMemory(void **aMemory, PRSize *aSize, bool aMayAllocate, PRSize aSizeNeeded)
+bool nsBidi::GetMemory(void **aMemory, size_t *aSize, bool aMayAllocate, size_t aSizeNeeded)
 {
   /* check for existing memory */
   if(*aMemory==NULL) {
diff --git a/layout/base/nsBidi.h b/layout/base/nsBidi.h
index 29888cacd72..d0e338912f5 100644
--- a/layout/base/nsBidi.h
+++ b/layout/base/nsBidi.h
@@ -823,7 +823,7 @@ protected:
   PRInt32 mLength;
 
   /** memory sizes in bytes */
-  PRSize mDirPropsSize, mLevelsSize, mRunsSize;
+  size_t mDirPropsSize, mLevelsSize, mRunsSize;
 
   /** allocated memory */
   DirProp* mDirPropsMemory;
@@ -860,7 +860,7 @@ private:
 
   void Init();
 
-  bool GetMemory(void **aMemory, PRSize* aSize, bool aMayAllocate, PRSize aSizeNeeded);
+  bool GetMemory(void **aMemory, size_t* aSize, bool aMayAllocate, size_t aSizeNeeded);
 
   void Free();
 
diff --git a/netwerk/protocol/http/nsHttpAuthCache.cpp b/netwerk/protocol/http/nsHttpAuthCache.cpp
index 0bd80f262bf..246f426feb8 100644
--- a/netwerk/protocol/http/nsHttpAuthCache.cpp
+++ b/netwerk/protocol/http/nsHttpAuthCache.cpp
@@ -190,7 +190,7 @@ nsHttpAuthCache::LookupAuthNode(const char *scheme,
 }
 
 void *
-nsHttpAuthCache::AllocTable(void *self, PRSize size)
+nsHttpAuthCache::AllocTable(void *self, size_t size)
 {
     return malloc(size);
 }
diff --git a/netwerk/protocol/http/nsHttpAuthCache.h b/netwerk/protocol/http/nsHttpAuthCache.h
index c9c75179028..e312b9a9007 100644
--- a/netwerk/protocol/http/nsHttpAuthCache.h
+++ b/netwerk/protocol/http/nsHttpAuthCache.h
@@ -221,7 +221,7 @@ private:
                                    nsCString  &key);
 
     // hash table allocation functions
-    static void*        AllocTable(void *, PRSize size);
+    static void*        AllocTable(void *, size_t size);
     static void         FreeTable(void *, void *item);
     static PLHashEntry* AllocEntry(void *, const void *key);
     static void         FreeEntry(void *, PLHashEntry *he, PRUintn flag);
diff --git a/rdf/base/src/nsRDFService.cpp b/rdf/base/src/nsRDFService.cpp
index 34113d3ccf4..65b88fa56c6 100644
--- a/rdf/base/src/nsRDFService.cpp
+++ b/rdf/base/src/nsRDFService.cpp
@@ -82,7 +82,7 @@ class BlobImpl;
 // XXX sigh, why were DefaultAllocTable et. al. declared static, anyway?
 
 static void *
-DataSourceAllocTable(void *pool, PRSize size)
+DataSourceAllocTable(void *pool, size_t size)
 {
     return PR_MALLOC(size);
 }
diff --git a/xpcom/base/nsMemoryImpl.cpp b/xpcom/base/nsMemoryImpl.cpp
index dc8523e9437..4d9dee82406 100644
--- a/xpcom/base/nsMemoryImpl.cpp
+++ b/xpcom/base/nsMemoryImpl.cpp
@@ -26,13 +26,13 @@ static nsMemoryImpl sGlobalMemory;
 NS_IMPL_QUERY_INTERFACE1(nsMemoryImpl, nsIMemory)
 
 NS_IMETHODIMP_(void*)
-nsMemoryImpl::Alloc(PRSize size)
+nsMemoryImpl::Alloc(size_t size)
 {
     return NS_Alloc(size);
 }
 
 NS_IMETHODIMP_(void*)
-nsMemoryImpl::Realloc(void* ptr, PRSize size)
+nsMemoryImpl::Realloc(void* ptr, size_t size)
 {
     return NS_Realloc(ptr, size);
 }
@@ -158,13 +158,13 @@ nsMemoryImpl::FlushEvent
 nsMemoryImpl::sFlushEvent;
 
 XPCOM_API(void*)
-NS_Alloc(PRSize size)
+NS_Alloc(size_t size)
 {
     return moz_xmalloc(size);
 }
 
 XPCOM_API(void*)
-NS_Realloc(void* ptr, PRSize size)
+NS_Realloc(void* ptr, size_t size)
 {
     return moz_xrealloc(ptr, size);
 }
diff --git a/xpcom/base/nsTraceRefcntImpl.cpp b/xpcom/base/nsTraceRefcntImpl.cpp
index 8ae61a663a8..eb4a540075e 100644
--- a/xpcom/base/nsTraceRefcntImpl.cpp
+++ b/xpcom/base/nsTraceRefcntImpl.cpp
@@ -155,7 +155,7 @@ AssertActivityIsLegal()
 // the BloatEntry.
 
 static void *
-DefaultAllocTable(void *pool, PRSize size)
+DefaultAllocTable(void *pool, size_t size)
 {
     return PR_MALLOC(size);
 }
diff --git a/xpcom/base/nsUUIDGenerator.cpp b/xpcom/base/nsUUIDGenerator.cpp
index 4c17e7614be..5871d19cc3f 100644
--- a/xpcom/base/nsUUIDGenerator.cpp
+++ b/xpcom/base/nsUUIDGenerator.cpp
@@ -40,9 +40,9 @@ nsUUIDGenerator::Init()
     /* initialize random number generator using NSPR random noise */
     unsigned int seed;
 
-    PRSize bytes = 0;
+    size_t bytes = 0;
     while (bytes < sizeof(seed)) {
-        PRSize nbytes = PR_GetRandomNoise(((unsigned char *)&seed)+bytes,
+        size_t nbytes = PR_GetRandomNoise(((unsigned char *)&seed)+bytes,
                                           sizeof(seed)-bytes);
         if (nbytes == 0) {
             return NS_ERROR_FAILURE;
@@ -120,7 +120,7 @@ nsUUIDGenerator::GenerateUUIDInPlace(nsID* id)
     setstate(mState);
 #endif
 
-    PRSize bytesLeft = sizeof(nsID);
+    size_t bytesLeft = sizeof(nsID);
     while (bytesLeft > 0) {
 #ifdef ANDROID
         long rval = arc4random();
@@ -137,8 +137,8 @@ nsUUIDGenerator::GenerateUUIDInPlace(nsID* id)
         src += sizeof(rval) - mRBytes;
 #endif
         PRUint8 *dst = ((PRUint8*) id) + (sizeof(nsID) - bytesLeft);
-        PRSize toWrite = (bytesLeft < mRBytes ? bytesLeft : mRBytes);
-        for (PRSize i = 0; i < toWrite; i++)
+        size_t toWrite = (bytesLeft < mRBytes ? bytesLeft : mRBytes);
+        for (size_t i = 0; i < toWrite; i++)
             dst[i] = src[i];
 
         bytesLeft -= toWrite;
diff --git a/xpcom/build/nsXPCOM.h b/xpcom/build/nsXPCOM.h
index 88ed01838a3..5546d7764be 100644
--- a/xpcom/build/nsXPCOM.h
+++ b/xpcom/build/nsXPCOM.h
@@ -214,7 +214,7 @@ NS_NewNativeLocalFile(const nsACString &path,
  * @note         This function is thread-safe.
  */
 XPCOM_API(void*)
-NS_Alloc(PRSize size);
+NS_Alloc(size_t size);
 
 /**
  * Reallocates a block of memory to a new size.
@@ -232,7 +232,7 @@ NS_Alloc(PRSize size);
  * allocation fails, the process aborts.
  */
 XPCOM_API(void*)
-NS_Realloc(void* ptr, PRSize size);
+NS_Realloc(void* ptr, size_t size);
 
 /**
  * Frees a block of memory. Null is a permissible value, in which case no
diff --git a/xpcom/build/nsXPCOMPrivate.h b/xpcom/build/nsXPCOMPrivate.h
index 5f616e4ab95..d55dce2658f 100644
--- a/xpcom/build/nsXPCOMPrivate.h
+++ b/xpcom/build/nsXPCOMPrivate.h
@@ -68,8 +68,8 @@ typedef bool       (* CStringGetIsVoidFunc)(const nsACString &);
 typedef nsresult   (* CStringToUTF16)(const nsACString &, nsCStringEncoding, nsAString &);
 typedef nsresult   (* UTF16ToCString)(const nsAString &, nsCStringEncoding, nsACString &);
 
-typedef void*      (* AllocFunc)(PRSize size);
-typedef void*      (* ReallocFunc)(void* ptr, PRSize size);
+typedef void*      (* AllocFunc)(size_t size);
+typedef void*      (* ReallocFunc)(void* ptr, size_t size);
 typedef void       (* FreeFunc)(void* ptr);
 
 typedef void       (* DebugBreakFunc)(PRUint32 aSeverity,
diff --git a/xpcom/glue/DeadlockDetector.h b/xpcom/glue/DeadlockDetector.h
index c1161725671..27157cf55a6 100644
--- a/xpcom/glue/DeadlockDetector.h
+++ b/xpcom/glue/DeadlockDetector.h
@@ -203,7 +203,7 @@ private:
         HashEntryArray mOrderedLT; // this <_o Other
     };
 
-    static void* TableAlloc(void* /*pool*/, PRSize size)
+    static void* TableAlloc(void* /*pool*/, size_t size)
     {
         return operator new(size);
     }
diff --git a/xpcom/glue/nsMemory.cpp b/xpcom/glue/nsMemory.cpp
index 8c6445fe4d3..a2790b74ba3 100644
--- a/xpcom/glue/nsMemory.cpp
+++ b/xpcom/glue/nsMemory.cpp
@@ -24,7 +24,7 @@ nsMemory::HeapMinimize(bool aImmediate)
 }
 
 NS_COM_GLUE void*
-nsMemory::Clone(const void* ptr, PRSize size)
+nsMemory::Clone(const void* ptr, size_t size)
 {
     void* newPtr = NS_Alloc(size);
     if (newPtr)
diff --git a/xpcom/glue/nsMemory.h b/xpcom/glue/nsMemory.h
index 04c11b22307..6ce3dce8745 100644
--- a/xpcom/glue/nsMemory.h
+++ b/xpcom/glue/nsMemory.h
@@ -35,14 +35,14 @@ public:
     static NS_HIDDEN_(void*) Alloc(size_t size)
         { return NS_Alloc(size); }
 
-    static NS_HIDDEN_(void*) Realloc(void* ptr, PRSize size)
+    static NS_HIDDEN_(void*) Realloc(void* ptr, size_t size)
         { return NS_Realloc(ptr, size); }
 
     static NS_HIDDEN_(void) Free(void* ptr)
         { NS_Free(ptr); }
 
     static NS_COM_GLUE nsresult   HeapMinimize(bool aImmediate);
-    static NS_COM_GLUE void*      Clone(const void* ptr, PRSize size);
+    static NS_COM_GLUE void*      Clone(const void* ptr, size_t size);
     static NS_COM_GLUE nsIMemory* GetGlobalMemoryService();       // AddRefs
 };
 
diff --git a/xpcom/glue/standalone/nsXPCOMGlue.cpp b/xpcom/glue/standalone/nsXPCOMGlue.cpp
index 704bb109fa5..de2da7b4c2f 100644
--- a/xpcom/glue/standalone/nsXPCOMGlue.cpp
+++ b/xpcom/glue/standalone/nsXPCOMGlue.cpp
@@ -408,7 +408,7 @@ NS_UTF16ToCString(const nsAString &aSrc, nsCStringEncoding aDestEncoding, nsACSt
 }
 
 XPCOM_API(void*)
-NS_Alloc(PRSize size)
+NS_Alloc(size_t size)
 {
     if (!xpcomFunctions.allocFunc)
         return nullptr;
@@ -416,7 +416,7 @@ NS_Alloc(PRSize size)
 }
 
 XPCOM_API(void*)
-NS_Realloc(void* ptr, PRSize size)
+NS_Realloc(void* ptr, size_t size)
 {
     if (!xpcomFunctions.reallocFunc)
         return nullptr;
diff --git a/xpcom/stub/nsXPComStub.cpp b/xpcom/stub/nsXPComStub.cpp
index 9d6282a5294..b74f5247db1 100644
--- a/xpcom/stub/nsXPComStub.cpp
+++ b/xpcom/stub/nsXPComStub.cpp
@@ -196,14 +196,14 @@ NS_GetTraceRefcnt(nsITraceRefcnt **result)
 
 #undef NS_Alloc
 EXPORT_XPCOM_API(void*)
-NS_Alloc(PRSize size)
+NS_Alloc(size_t size)
 {
   return NS_Alloc_P(size);
 }
 
 #undef NS_Realloc
 EXPORT_XPCOM_API(void*)
-NS_Realloc(void* ptr, PRSize size)
+NS_Realloc(void* ptr, size_t size)
 {
   return NS_Realloc_P(ptr, size);
 }