Suddenly, it started to miss its borders of the main window, being maximized always and didn't react on any changes like shrink, move, resize, maximize.
It covers the toolbar completely and when you open for example its addons window it doesn't show up, because it is hidden beneath the main window.
BTW, I use a two monitor system, and the main window always shows up on the left screen.
Haven't tried it with a single monitor.
Finally, I found a fix of this problem:
- Close Thunderbird
- Edit by hand the file ~/.mozilla-thunderbird/xyz1234.default/localstore.rdf in your default profile directory.
- Search the entry
- Change the width, height and screenXY entries to the ones you see here
- Restart Thunderbird and it should have its borders again.