firefox -safemode or -safe-mode

then a new window should show to enter safe mode or Reset, hit the right keyboard key, and press enter on "Reset Firefox". No mouse needed.

XP

   Open the Command Prompt:

       Windows Vista: Click on the Start Menu, click on All Programs, click on the Mozilla Firefox folder and click on Mozilla Firefox (Safe Mode).

       ff_vistastartmenu.png (+)

       Windows XP and Earlier: Click on the Start Menu, click on All Programs, click on the Mozilla Firefox folder and click on Mozilla Firefox (Safe Mode).

       ff_xpstartmenu.png (+)

   In the Firefox Safe Mode window, check the Reset all user preferences to Firefox defaults box and click on Make Changes and Restart.

   ff_safemode.png (+)

   Firefox will automatically restart and your settings will be reset to defaults.

Thanks @Waka_Flock_Flame, but I have no keyboard either. It's an server and xvfb is just a virtual X-Server.

Thanks @Mohammed.samad, but I use Linux.

Otherwise, without a keyboard and mouse, you can just reinstall and delete the ./mozilla folder.

Thanks! Remove and install didn't help, but the trick is removing the ./mozilla folder. Thanks a lot!!!

You may have to check the ~/.cache/mozilla/firefox folder as well that current Firefox versions use for the disk cache.