diff --git a/xpcom/base/nsDebugImpl.cpp b/xpcom/base/nsDebugImpl.cpp
index c5f1204276f..afc3fbe4947 100644
--- a/xpcom/base/nsDebugImpl.cpp
+++ b/xpcom/base/nsDebugImpl.cpp
@@ -433,6 +433,7 @@ NS_DebugBreak(uint32_t aSeverity, const char* aStr, const char* aExpr,
       nsTraceRefcnt::WalkTheStack(stderr);
 #endif // !defined(MOZILLA_XPCOMRT_API)
       // Fall through to abort
+      MOZ_FALLTHROUGH;
 
     case NS_ASSERT_ABORT:
       Abort(buf.buffer);
@@ -607,4 +608,3 @@ NS_ABORT_OOM(size_t aSize)
 #endif
   MOZ_CRASH();
 }
-
diff --git a/xpcom/ds/nsCheapSets.h b/xpcom/ds/nsCheapSets.h
index e61febffe5a..1401bc5f1a2 100644
--- a/xpcom/ds/nsCheapSets.h
+++ b/xpcom/ds/nsCheapSets.h
@@ -135,7 +135,8 @@ nsCheapSet<EntryType>::Put(const KeyType aVal)
       mUnion.table = table;
       mState = MANY;
     }
-    // Fall through.
+    MOZ_FALLTHROUGH;
+
     case MANY:
       mUnion.table->PutEntry(aVal);
       return NS_OK;
diff --git a/xpcom/ds/nsPersistentProperties.cpp b/xpcom/ds/nsPersistentProperties.cpp
index 8f62bec775b..f783e21d82e 100644
--- a/xpcom/ds/nsPersistentProperties.cpp
+++ b/xpcom/ds/nsPersistentProperties.cpp
@@ -227,7 +227,7 @@ nsPropertiesParser::ParseValueCharacter(char16_t aChar, const char16_t* aCur,
             aTokenStart = aCur + 1;
             break;
           }
-        // no break
+          MOZ_FALLTHROUGH;
 
         case '\r':
           // we're done! We have a key and value
diff --git a/xpcom/ds/nsVariant.cpp b/xpcom/ds/nsVariant.cpp
index 1c4ad76289c..fc4c6d11ff0 100644
--- a/xpcom/ds/nsVariant.cpp
+++ b/xpcom/ds/nsVariant.cpp
@@ -356,7 +356,8 @@ CloneArray(uint16_t aInType, const nsIID* aInIID,
       if (aOutIID) {
         *aOutIID = *aInIID;
       }
-    // fall through...
+      MOZ_FALLTHROUGH;
+
     case nsIDataType::VTYPE_INTERFACE: {
       memcpy(*aOutValue, aInValue, allocSize);
 
@@ -759,7 +760,7 @@ nsDiscriminatedUnion::ToString(nsACString& aOutString) const
     case nsIDataType::VTYPE_WSTRING_SIZE_IS:
     case nsIDataType::VTYPE_WCHAR:
       NS_ERROR("ToString being called for a string type - screwy logic!");
-      // fall through...
+      MOZ_FALLTHROUGH;
 
     // XXX We might want stringified versions of these... ???
 
@@ -1654,10 +1655,12 @@ nsDiscriminatedUnion::Traverse(nsCycleCollectionTraversalCallback& aCb) const
             NS_CYCLE_COLLECTION_NOTE_EDGE_NAME(aCb, "mData[i]");
             aCb.NoteXPCOMChild(*p);
           }
+          break;
         }
         default:
           break;
       }
+      break;
     default:
       break;
   }
diff --git a/xpcom/glue/nsTextFormatter.cpp b/xpcom/glue/nsTextFormatter.cpp
index c361ca9f1ef..cceec274c59 100644
--- a/xpcom/glue/nsTextFormatter.cpp
+++ b/xpcom/glue/nsTextFormatter.cpp
@@ -351,14 +351,14 @@ cvt_f(SprintfState* aState, double aDouble, int aWidth, int aPrec,
       break;
     case 'E':
       exp = 'E';
-    // no break
+      MOZ_FALLTHROUGH;
     case 'e':
       numdigits = aPrec + 1;
       mode = 2;
       break;
     case 'G':
       exp = 'E';
-    // no break
+      MOZ_FALLTHROUGH;
     case 'g':
       if (aPrec == 0) {
         aPrec = 1;
diff --git a/xpcom/io/nsWildCard.cpp b/xpcom/io/nsWildCard.cpp
index 9b8ab8112b5..9125cbbb8f6 100644
--- a/xpcom/io/nsWildCard.cpp
+++ b/xpcom/io/nsWildCard.cpp
@@ -81,7 +81,7 @@ _valid_subexp(const T* aExpr, T aStop1, T aStop2)
           return INVALID_SXP;
         }
         ++tld;
-      /* fall through */
+        MOZ_FALLTHROUGH;
       case '*':
       case '?':
       case '$':
@@ -397,7 +397,7 @@ _shexp_match(const T* aStr, const T* aExpr, bool aCaseInsensitive,
         return ABORTED;
       case '\\':
         ++y;
-      /* fall through */
+        MOZ_FALLTHROUGH;
       default:
         if (aCaseInsensitive) {
           if (::upper(aStr[x]) != ::upper(aExpr[y])) {
@@ -414,7 +414,6 @@ _shexp_match(const T* aStr, const T* aExpr, bool aCaseInsensitive,
   return (aStr[x] ? NOMATCH : MATCH);
 }
 
-
 template<class T>
 static int
 ns_WildCardMatch(const T* aStr, const T* aXp, bool aCaseInsensitive)