Comment 35 for bug 196058

Marco's note was the key for me. In my case some conflict between Compiz and Mate caused my desktop to go completely crazy. One of the effects was to make every application go full screen. After cleaning up the mess, all the apps went back to more-or-less normal but Thunderbird. I had already tried the localstore.rdf method without success. But Marco's note about using xulstore.json did the trick!

Until then, while I tried to get this working, the only way I could get to my Panel was by hitting F12, which brought up the miniterminal and coincidentally showed the Panel. But then I could only minimize Thunderbird. (And I ended up with a dozen or so miniterminal windows...) The right-click options on the Thunderbird button in the task list did not show Thunderbird as maximized. So the easiest thing was to just use File->exit in Thunderbird each time I tried things.