Safe Mode is a special Firefox execution mode that can be used to troubleshoot issues in Firefox. In Safe Mode, you can reset some settings or disable add-ons that might be the source of the issue. By comparing Firefox behavior in normal mode to its behavior in Safe Mode, where various items are disabled, you may be able to diagnose issues.

How to start Firefox in Safe Mode

Click the menu button , click Help, select Troubleshoot Mode… and click Restart in the Restart Firefox in Troubleshoot Mode? dialog.

Safe Mode window

You now have three options:

• Clicking the Exit button cancels your attempt to get into Firefox's Safe Mode.
• Clicking the Continue In Safe Mode button starts Firefox in its Safe Mode. While you are in Safe Mode, your extensions and themes will be disabled, and any toolbar customizations will be reverted back to their defaults. These changes are temporary - 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.
• The Make Changes and Restart button is only enabled if you select one of the boxes above it.

You can elect to make the following changes:

• Disable all add-ons: If you select this box and then click Make Changes and Restart then Firefox will start back up in its normal mode, except all your extensions, themes and plugins will be disabled (not uninstalled). You can undo this change by enabling each extension and plugin listed in the Add-ons Manager. See this article for general information about using the Add-ons Manager to disable and enable add-ons.
• Reset toolbars and controls: If you select this box and then click Make Changes and Restart then Firefox will start back up in its normal mode, except any customizations you have made will be lost, such as window size or position, toolbar buttons, and other toolbar customizations. To undo this change, you will need to customize these settings again. See Customize Firefox controls, buttons and toolbars for help with toolbar settings.
• Delete all bookmarks except for backups: If you select this box and then click Make Changes and Restart then Firefox will delete your current bookmarks and replace them with the default set (this item is labeled Reset bookmarks to Firefox defaults in Firefox 3.5). Except for bookmarks added in the current session which will be lost, you can undo this change by restoring the last bookmark backup. See Restore bookmarks from backup or move them to another computer for more help.
• Reset all user preferences to Firefox defaults: If you select this box and then click Make Changes and Restart then changes you have made to Firefox options and preferences will be changed back to the default settings. This change will be hard to undo, since you may not remember all of the settings you have changed.
• Restore default search engines: If you select this box and then click Make Changes and Restart then any of the default search engines you deleted will be restored. To undo this change, simply go to the Search Bar and remove the search engines you don't want.

Exiting Safe Mode

1. Click the Firefox menu and select Exit.Click the Firefox menu at the top of the screen and select Quit Firefox.Click the Firefox menu and select Quit.
2. Start Firefox as you normally would.
   • If Firefox keeps starting in Safe Mode, see Firefox is stuck in Troubleshoot Mode.

Based on information from Safe mode (mozillaZine KB)