MesQuilla

I'm not sure that the "append to subject" (which I see is confusingly named) would actually work on a forward action. Have you tried it? It only changes the message subject in the local database, not in the message itself, and code that reads the message directly to get the subject won't see it.

As to the customization, it would be really easy for you to modify the code yourself to do exactly what you want. The extension .xpi file is just a zip file, unzip an open the file content/filtaquilla.js There you will find a function " _mimeAppend". Just change this line:

appendedSubject = utf8Append decodedSubject;

to:

appendedSubject = decodedSubject utf8Append;

Of course then you'd have the issue of maintaining this with new versions, which might be a pain … I'll make a note of this in my bug database, and try to add some sort of workaround in future versions.