If Firefox is open, you can restart in Firefox Safe Mode from the Help menu:
- In Firefox 29.0 and above, click the menu button , click Help and select Restart with Add-ons Disabled.
- In previous Firefox versions, click on the Firefox button at the top left of the Firefox window and click on Help (or click on Help in the Menu bar, if you don't have a Firefox button) then click on Restart with Add-ons Disabled.
If Firefox is not running, you can start Firefox in Safe Mode as follows:
- On Windows: Hold the Shift key when you open the Firefox desktop or Start menu shortcut.
- On Mac: Hold the option key while starting Firefox.
- On Linux: Quit Firefox, go to your Terminal and run firefox -safe-mode
(you may need to specify the Firefox installation path e.g. /usr/lib/firefox)
When the Firefox Safe Mode window appears, select "Start in Safe Mode".
If the issue is not present in Firefox Safe Mode, your problem is probably caused by an extension, theme, or hardware acceleration. Please follow the steps in the Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems article to find the cause.
To exit Firefox Safe Mode, just close Firefox and wait a few seconds before opening Firefox for normal use again.
The user.js file does not exist by default. If you create the file, forget it, if you have not create the file then delete the User.js file, to see if the problem goes away (some programs create and write in the file also).
You can cleanup the prefs to have a fresh start and delete possible user.js and numbered prefs-##.js files and rename (or delete) the prefs.js file to reset all prefs to the default value including prefs set via user.js and prefs that are no longer supported in the current Firefox release.
You can use this button to go to the currently used Firefox profile folder:
- Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder)