Comment 7 for bug 196058

I found a very simple and safe workaround.

1) Open Tbird.
2) Change Windowmanger (e.g. compiz to metacity). You can do this easily utilizin Compiz Fusion Icon.
3) Now should be able to resize the window.
4) Change back to your original Windowmanager.

This worked fine for me, hope it will work for others, too.