Comment 13 for bug 196058

Exactly, the problem lies in the ~/.mozilla-thunderbird/xyz1234.default/localstore.rdf settings file.

I can reproduce the problem quickly on Ubuntu Hardy with Dual Monitor.
My right monitor has a higher resolution than the left one.
I start Thunderbird, maximize it on the right monitor (that with the higher resolution), close Thunderbird.
When I open Thunderbird again, it opens in fullscreen on the left monitor and gets stucked in this setup. Nothing helps to change that outfit.

You can restore the windows border with these steps:
# Close Thunderbird
# Edit the file ~/.mozilla-thunderbird/xyz1234.default/localstore.rdf
# Search the entry
  <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.