Troubleshoot Mode is a special Firefox mode that can be used to diagnose and fix problems. (Troubleshoot Mode was known as Safe Mode in previous versions of Firefox.)
When you start Firefox in Troubleshoot Mode, it temporarily disables add-ons (extensions and themes), turns off hardware acceleration and certain other features, and ignores some customizations (see below to learn more). Comparing Firefox’s behavior in normal mode to its behavior in Troubleshoot Mode may help pinpoint the cause of the problem.
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How to start Firefox in Troubleshoot Mode
Click the menu button 
, click Help, select Troubleshoot Mode… and click Restart in the Restart Firefox in Troubleshoot Mode? dialog.Note: You can also start Firefox in Troubleshoot Mode by holding down the shift key while starting Firefox.
Troubleshoot Mode window
You will have two options:
- Clicking the Open button will start Firefox with the default theme, your extensions will be disabled, and some other features and customizations will be turned off. These changes are temporary. When you leave Troubleshoot Mode and start Firefox normally, your add-ons and other settings will return to the state they were in before you entered Troubleshoot Mode.
- Clicking the Refresh Firefox button will restore Firefox to its factory default state while saving your essential information. These changes are permanent.
Diagnosing problems in Troubleshoot Mode
After you start Firefox in Troubleshoot Mode, you’ll want to test its behavior and see if the problem goes away.
The problem happens in Troubleshoot Mode
If the problem still happens in Troubleshoot Mode, it is not being caused by an extension or theme. Other possible causes include changes made to Firefox preference settings, which are not disabled in Troubleshoot Mode.
The problem does not happen in Troubleshoot Mode
If the problem does not happen in Troubleshoot Mode, the most likely cause is an extension, theme or hardware acceleration. See this article for solutions:
Exiting Troubleshoot Mode
- Click the Firefox menu and select Exit.
- Start Firefox as you normally would.
More information (Advanced users)
In addition to disabling add-ons (extensions and themes) and turning off hardware acceleration, Firefox Troubleshoot Mode disables Site Isolation, the JavaScript Just-in-time (JIT) compiler and ignores the optional userContent.css and userChrome.css files (if present).
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