diff --git a/mozilla/xpcom/string/public/nsCharTraits.h b/mozilla/xpcom/string/public/nsCharTraits.h
index 00f05d19df8..57384405bcb 100644
--- a/mozilla/xpcom/string/public/nsCharTraits.h
+++ b/mozilla/xpcom/string/public/nsCharTraits.h
@@ -200,6 +200,28 @@ struct nsCharTraits<PRUnichar>
         return 0;
       }
 
+    // this version assumes that s2 is null-terminated and s1 has length n.
+    // if s1 is shorter than s2 then we return -1; if s1 is longer than s2,
+    // we return 1.
+    static
+    int
+    compareASCIINullTerminated( const char_type* s1, size_t n, const char* s2 )
+      {
+        for ( ; n--; ++s1, ++s2 )
+          {
+            if ( !*s2 )
+              return 1;
+            NS_ASSERTION(!(*s2 & ~0x7F), "Unexpected non-ASCII character");
+            if ( !eq_int_type(to_int_type(*s1), to_int_type(*s2)) )
+              return to_int_type(*s1) - to_int_type(*s2);
+          }
+
+        if ( *s2 )
+          return -1;
+
+        return 0;
+      }
+
     static
     size_t
     length( const char_type* s )
@@ -364,6 +386,30 @@ struct nsCharTraits<char>
         return compare(s1, s2, n);
       }
 
+    // this version assumes that s2 is null-terminated and s1 has length n.
+    // if s1 is shorter than s2 then we return -1; if s1 is longer than s2,
+    // we return 1.
+    static
+    int
+    compareASCIINullTerminated( const char_type* s1, size_t n, const char* s2 )
+      {
+        // can't use strcmp here because we don't want to stop when s1
+        // contains a null
+        for ( ; n--; ++s1, ++s2 )
+          {
+            if ( !*s2 )
+              return 1;
+            NS_ASSERTION(!(*s2 & ~0x7F), "Unexpected non-ASCII character");
+            if ( *s1 != *s2 )
+              return to_int_type(*s1) - to_int_type(*s2);
+          }
+
+        if ( *s2 )
+          return -1;
+
+        return 0;
+      }
+
     static
     size_t
     length( const char_type* s )
diff --git a/mozilla/xpcom/string/public/nsTAString.h b/mozilla/xpcom/string/public/nsTAString.h
index 8eb4fcf3a12..a95d61ccbe3 100644
--- a/mozilla/xpcom/string/public/nsTAString.h
+++ b/mozilla/xpcom/string/public/nsTAString.h
@@ -198,10 +198,18 @@ class nsTAString_CharT
       NS_COM PRBool Equals( const char_type*, const comparator_type& ) const;
 
         /**
-         * An efficient comparison with ASCII that can be used even for
-         * wide strings.
+         * An efficient comparison with ASCII that can be used even
+         * for wide strings. Call this version when you know the
+         * length of 'data'.
          */
       NS_COM PRBool EqualsASCII( const char* data, size_type len ) const;
+        /**
+         * An efficient comparison with ASCII that can be used even
+         * for wide strings. Call this version when 'data' is
+         * null-terminated.
+         */
+      NS_COM PRBool EqualsASCII( const char* data ) const;
+
     // EqualsLiteral must ONLY be applied to an actual literal string.
     // Do not attempt to use it with a regular char* pointer, or with a char
     // array variable.
@@ -210,7 +218,7 @@ class nsTAString_CharT
 #ifdef NS_DISABLE_LITERAL_TEMPLATE
       inline PRBool EqualsLiteral( const char* str ) const
         {
-          return EqualsASCII(str, strlen(str));
+          return EqualsASCII(str);
         }
 #else
       template<int N>
diff --git a/mozilla/xpcom/string/public/nsTSubstring.h b/mozilla/xpcom/string/public/nsTSubstring.h
index a62311bafdd..35727c43f46 100644
--- a/mozilla/xpcom/string/public/nsTSubstring.h
+++ b/mozilla/xpcom/string/public/nsTSubstring.h
@@ -215,7 +215,19 @@ class nsTSubstring_CharT : public nsTAString_CharT
       NS_COM PRBool Equals( const char_type* data ) const;
       NS_COM PRBool Equals( const char_type* data, const comparator_type& comp ) const;
 
+        /**
+         * An efficient comparison with ASCII that can be used even
+         * for wide strings. Call this version when you know the
+         * length of 'data'.
+         */
       NS_COM PRBool EqualsASCII( const char* data, size_type len ) const;
+        /**
+         * An efficient comparison with ASCII that can be used even
+         * for wide strings. Call this version when 'data' is
+         * null-terminated.
+         */
+      NS_COM PRBool EqualsASCII( const char* data ) const;
+
     // EqualsLiteral must ONLY be applied to an actual literal string.
     // Do not attempt to use it with a regular char* pointer, or with a char
     // array variable.
@@ -224,7 +236,7 @@ class nsTSubstring_CharT : public nsTAString_CharT
 #ifdef NS_DISABLE_LITERAL_TEMPLATE
       inline PRBool EqualsLiteral( const char* str ) const
         {
-          return EqualsASCII(str, strlen(str));
+          return EqualsASCII(str);
         }
 #else
       template<int N>
diff --git a/mozilla/xpcom/string/src/nsTAString.cpp b/mozilla/xpcom/string/src/nsTAString.cpp
index bd0aa3c0bf4..19efa706cf8 100644
--- a/mozilla/xpcom/string/src/nsTAString.cpp
+++ b/mozilla/xpcom/string/src/nsTAString.cpp
@@ -114,6 +114,15 @@ nsTAString_CharT::EqualsASCII( const char* data, size_type len ) const
     return ToSubstring().EqualsASCII(data, len);
   }
 
+PRBool
+nsTAString_CharT::EqualsASCII( const char* data ) const
+  {
+    if (mVTable == obsolete_string_type::sCanonicalVTable)
+      return AsSubstring()->EqualsASCII(data);
+
+    return ToSubstring().EqualsASCII(data);
+  }
+
 PRBool
 nsTAString_CharT::IsVoid() const
   {
diff --git a/mozilla/xpcom/string/src/nsTSubstring.cpp b/mozilla/xpcom/string/src/nsTSubstring.cpp
index d457959290f..91bd5cd0e36 100644
--- a/mozilla/xpcom/string/src/nsTSubstring.cpp
+++ b/mozilla/xpcom/string/src/nsTSubstring.cpp
@@ -561,6 +561,12 @@ nsTSubstring_CharT::EqualsASCII( const char* data, size_type len ) const
     return mLength == len && char_traits::compareASCII(mData, data, len) == 0;
   }
 
+PRBool
+nsTSubstring_CharT::EqualsASCII( const char* data ) const
+  {
+    return char_traits::compareASCIINullTerminated(mData, mLength, data) == 0;
+  }
+
 nsTSubstring_CharT::size_type
 nsTSubstring_CharT::CountChar( char_type c ) const
   {