diff --git a/mozilla/xpcom/string/public/nsTSubstring.h b/mozilla/xpcom/string/public/nsTSubstring.h
index 2c19137490f..81158d034f6 100644
--- a/mozilla/xpcom/string/public/nsTSubstring.h
+++ b/mozilla/xpcom/string/public/nsTSubstring.h
@@ -300,7 +300,7 @@ class nsTSubstring_CharT : public nsTAString_CharT
          * assignment
          */
 
-      void Assign( char_type c )                                                                { Assign(&c, 1); }
+      NS_COM void NS_FASTCALL Assign( char_type c );
       NS_COM void NS_FASTCALL Assign( const char_type* data, size_type length = size_type(-1) );
       NS_COM void NS_FASTCALL Assign( const self_type& );
       NS_COM void NS_FASTCALL Assign( const substring_tuple_type& );
@@ -341,7 +341,7 @@ class nsTSubstring_CharT : public nsTAString_CharT
          * buffer manipulation
          */
 
-             void Replace( index_type cutStart, size_type cutLength, char_type c )               { Replace(cutStart, cutLength, &c, 1); }
+      NS_COM void NS_FASTCALL Replace( index_type cutStart, size_type cutLength, char_type c );
       NS_COM void NS_FASTCALL Replace( index_type cutStart, size_type cutLength, const char_type* data, size_type length = size_type(-1) );
              void Replace( index_type cutStart, size_type cutLength, const self_type& str )      { Replace(cutStart, cutLength, str.Data(), str.Length()); }
       NS_COM void NS_FASTCALL Replace( index_type cutStart, size_type cutLength, const substring_tuple_type& tuple );
diff --git a/mozilla/xpcom/string/src/nsTSubstring.cpp b/mozilla/xpcom/string/src/nsTSubstring.cpp
index 1eb89b2de3b..f2872c2ff67 100644
--- a/mozilla/xpcom/string/src/nsTSubstring.cpp
+++ b/mozilla/xpcom/string/src/nsTSubstring.cpp
@@ -283,6 +283,15 @@ nsTSubstring_CharT::EnsureMutable()
 
 // ---------------------------------------------------------------------------
 
+  // This version of Assign is optimized for single-character assignment.
+void
+nsTSubstring_CharT::Assign( char_type c )
+  {
+    if (ReplacePrep(0, mLength, 1))
+      *mData = c;
+  }
+
+
 void
 nsTSubstring_CharT::Assign( const char_type* data, size_type length )
   {
@@ -421,6 +430,17 @@ nsTSubstring_CharT::Adopt( char_type* data, size_type length )
   }
 
 
+  // This version of Replace is optimized for single-character replacement.
+void
+nsTSubstring_CharT::Replace( index_type cutStart, size_type cutLength, char_type c )
+  {
+    cutStart = PR_MIN(cutStart, Length());
+
+    if (ReplacePrep(cutStart, cutLength, 1))
+      mData[cutStart] = c;
+  }
+
+
 void
 nsTSubstring_CharT::Replace( index_type cutStart, size_type cutLength, const char_type* data, size_type length )
   {