Resetting Firefox will not delete bookmarks, or saved passwords, but one cannot take guarantee of this. It might be best to back up your Mozilla Firefox bookmarks first and then implement the following steps:
• Click on the three stacked lines icon present at the top right of the browser window.
• Then select the question-mark icon at the bottom of the drop-down menu.
• Now click on the 'Troubleshooting information' present in the slide-out menu.
• You have to click on the 'Reset Firefox' button.
• Then choose the 'Reset Firefox' in the confirmation pop-up window.
If the error persists, then you need to call mozilla firefox customer support to speak with the professional experts who are there to help you in fixing the issue.

Visit here: How to recover Mozilla Firefox to default settings

Also read: Fix Slowness Crashing Error Messages Firefox