Mac OS X – Firefox preferences and windows open off screen
If you use two monitors and then remove one, for example from your laptop, you may notice that Firefox thinks the other monitor is still there and opens it’s preferences and windows off screen. To fix this without re-connecting the monitor:
1) Quit Firefox
2) Hold down the
option key while you start Firefox
3) Select “Reset toolbars and controls” and click “Make Changes and Restart”.