don't know how I omitted this last night, particularly since mscott tested. must shift the bit in the mask to get a new position. This should fix the string hang

string/public/nsAReadableString.h

@@ -1284,7 +1284,7 @@ inline
 nsPromiseConcatenation<CharT>
 operator+( const nsPromiseConcatenation<CharT>& lhs, const basic_nsAReadableString<CharT>& rhs )
   {
-    return nsPromiseConcatenation<CharT>(lhs, rhs, lhs.GetFragmentIdentifierMask());
+    return nsPromiseConcatenation<CharT>(lhs, rhs, lhs.GetFragmentIdentifierMask()<<1);
   }
 
 template <class CharT>

xpcom/ds/nsAReadableString.h

@@ -1284,7 +1284,7 @@ inline
 nsPromiseConcatenation<CharT>
 operator+( const nsPromiseConcatenation<CharT>& lhs, const basic_nsAReadableString<CharT>& rhs )
   {
-    return nsPromiseConcatenation<CharT>(lhs, rhs, lhs.GetFragmentIdentifierMask());
+    return nsPromiseConcatenation<CharT>(lhs, rhs, lhs.GetFragmentIdentifierMask()<<1);
   }
 
 template <class CharT>

xpcom/string/public/nsAReadableString.h

@@ -1284,7 +1284,7 @@ inline
 nsPromiseConcatenation<CharT>
 operator+( const nsPromiseConcatenation<CharT>& lhs, const basic_nsAReadableString<CharT>& rhs )
   {
-    return nsPromiseConcatenation<CharT>(lhs, rhs, lhs.GetFragmentIdentifierMask());
+    return nsPromiseConcatenation<CharT>(lhs, rhs, lhs.GetFragmentIdentifierMask()<<1);
   }
 
 template <class CharT>