Comment 28 for bug 196058

Well, I just had the same issue in Ubuntu 13.04, of course it didn't occur after a distro upgrade, I believe Thunderbird just updated, and I noticed compiz also crashed and restarted (updated @ night while I was asleep, and woke to find the error report about the crash, and all my windows were in a single workspace, which happens when compiz restarts). Then when I went to open Thunderbird it was fullscreen.

To fix this you can actually just delete localstore.rdf in the Thunderbird profile, if you want to be on the safe side just rename it to localstore.rdf.bak or localstore.rdf.old or something, then when you restart it and it creates the new file, and you're comfortable with it, delete the old file.

It is totally safe to delete though, because if it doesn't exist when Thunderbird opens it automatically creates it with default values (which is basically no values at all). Then when you change the size of the window to what you prefer, it will update the file with the appropriate values to remember how the window was.

And this is what's so amazing about text based configuration, in Window$ with a program that uses the registry for everything, you'd have to find the specific registry entry, and deleting it would likely break the program (and maybe even cause BSoD), so you'd likely have to find out what the key is and what value it's supposed to have.