Button is misplaced after exiting fullscreen mode

[adalucinet]

STR:

1. Navigate to http://chuclone.com/
2. Click on the Fullscreen button.
3. Exit Fullscreen.

Expected results: Fullscreen button maintains its position after exiting fullscreen.

Actual results: Fullscreen button changes position.

Notes:

1. Reproducible on all platforms. It isn't an issue on Chrome nor Opera.
2. Reproducible with Firefox 22 beta 3 (Build ID: 20130528181031), FF 21.0 (Build ID: 20130512193902), latest Nightly (Build ID: 20130602031240) and latest Aurora (Build ID: 20130603004018).
3. This issue is not a regression: it reproduces back to Firefox 6.0 (when Webgl is first supported on Firefox).