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Safe Mode is a special Firefox mode that can be used to troubleshoot and fix problems. Safe Mode temporarily turns off hardware acceleration, resets some settings, and disables add-ons (extensions and themes) that might be causing problems. By comparing Firefox behavior in normal mode to its behavior in Safe Mode you may be able to pinpoint the cause of the problem.

Note: Windows users may be familiar with a Safe Mode for Windows. Firefox Safe Mode has no connection to Windows Safe Mode.

The Refresh Firefox feature can fix many issues by restoring Firefox to its default state while saving your essential information. Consider using it before going through a lengthy troubleshooting process.

How to start Firefox in Safe Mode

Click the menu button , click help Click the menu button , click Help and select Restart with Add-ons Disabled…. Firefox will start up with the Firefox Safe Mode dialog.

Note: You can also start Firefox in Safe Mode by holding down the shift key while starting Firefox.holding down the option key while starting Firefox.quitting Firefox and then going to your Terminal and running: firefox -safe-mode
You may need to specify the Firefox installation path (e.g. /usr/lib/firefox)

Safe Mode window

You now have two options:

Clicking the Start in Safe Mode button will temporarily disable your extensions and themes, turn off hardware acceleration and reset toolbar and button customizations. When you leave Safe Mode and start Firefox up normally, your extensions, themes, and settings will return to the state they were in before you entered Safe Mode.
Clicking the Refresh Firefox button will restore Firefox to its factory default state while saving your essential information. See Refresh Firefox - reset add-ons and settings for more information about this option.

Troubleshooting problems in Safe Mode

Once Firefox is in Safe Mode, you'll want to test its behavior and see if the problem goes away.

The problem happens in Safe Mode

If the problem still happens in Safe Mode, it is not being caused by an extension or theme. Other possible causes could be plugins or changes made to Firefox preference settings, which are not disabled in Safe Mode. See the following articles for solution:

The problem does not happen in Safe Mode

If the problem does not happen in Safe Mode, it is most likely because of an extension, theme or hardware acceleration.

See the article Troubleshoot extensions, themes and hardware acceleration issues to solve common Firefox problems for solutions.

Exiting Safe Mode

Click the menu button and then click ExitQuit .
Click the Firefox menu , then click Exit. Click the Firefox menu at the top of the screen, then click Quit Firefox.Click the Firefox menu , then click Quit.
Start Firefox as you normally would.
- If Firefox keeps starting in Safe Mode, see Firefox is stuck in Safe Mode.

Based on information from Safe mode (mozillaZine KB)

Firefox