I tried the Safe Mode several times now. Interestingly, the bug goes away after starting Firefox in Safe Mode, but re-appears after some time.
The same happens if I switch back to the usual mode: it starts again and after some time, the bug appears.
I have not found any pattern in the re-appearance so far. Switching between a different program and Firefox several times does not reproduce the error - everything works fine, until at some randomly appearing point, the bug appears again, and does not go away until a restart of Firefox. This point is at some time between a few minutes and 1-2 hours. (probably, there is a trigger for the bug, but I couldn't find it yet)
The bug also appears in the Nightly version in the very same way: After a fresh restart, it cannot be reproduced. Only at some time, the bug starts to appear and persists, I.e. within any change from another program in a maximized window to Firefox in a previously maximized window, the Firefox window gets shrunk to "normal mode" with the size parameters written in the .xulstore file