How to Start Firefox in Safe Mode?
In the current version of Firefox, the “Safe Mode” menu item has been replaced by “Restart with Add-on Disable”. It still receives the same “safe mode” effect if you are experiencing a bug or problem when you open Safe Mode disappears, then the problem is probably due to your addon.
Start Firefox in Safe Mode Using Firefox Menu
- Open Mozilla Firefox This method shows two ways to restart Firefox in “safe mode”, with disabling all your add-ons.
- Click on the menu icon This icon looks like three horizontal grey bars, it is located on the right-hand side of your screen, as your address bar (URL) on the same tab. When you click it, you should open a small menu. 
Alternatively, disable the add-on in the top menu, click on Help → Restart, and go to “Continue in safe mode”.
- Open Help Click the small, fourth question mark icon at the bottom of the menu that you just opened. This will open the help menu when you click on the question mark, a list of options should be visible.
- Disable click with Add-on Disabled Select Add Restart with Add-on Disabled Firefox will immediately close and reopen. After using Firefox, take a screenshot or bookmark this page to continue using these instructions.
- Click Continue in the safe mode. When Firefox restarts, a pop-up should appear. To enter safe mode, click continue in safe mode.
- Disable add-ons one by one if the Safe Mode fixes your problem, then possibly due to the add-on, turn off Firefox and open it normally again. Use the top menu to access the tool → Add-on, and disable the add-on that you think might cause problems. Continue restarting Firefox and disable individual add-ons unless the bug is isolated.
- Reset Firefox completely if nothing works If the safe mode does not fix your problem, then you can remove Firefox’s default status and delete all your favourite preferences, bookmarks, passwords and add-ons. Repeat the steps above to restart Firefox in safe mode. This time, instead of “Continue in safe mode”, press Reset Firefox.
If you see a list of checkboxes instead, you are using Firefox version 14 or below. You can see all these boxes for a similar effect in 2nd option, but updating the latest Firefox can be an easy way to fix some bugs.
If you see only the “Continue in safe mode” option, then try switching to the default profile and try again 3rd option.
Your data will be saved in a new folder called “old firefox data” If you are creating a new Firefox profile, you can move harmless data over option (4).