You can restore the windows border with these steps:
<RDF:Description RDF:about=“chrome:messenger/content/messenger.xul#messengerWindow” width=“800” height=“600” sizemode=“normal” screenX=“5” screenY=“5” /> ====== Change the width, height and screenXY entries to the ones you see here ====== ====== Restart Thunderbird and it should have its borders again. ======