Try: https://support.mozilla.org/en-US/kb/refresh-firefox-reset-add-ons-and-settings

Though I would think it maybe more of a issue with your Avast security as some of these can cause issues if not setup properly for Firefox/

Check the Avast forum to see if others have that issue and what they suggest.

Please let us know if this solved your issue or if need further assistance.

In the event that the passwords information is lost, you should record all information in a separate text file somewhere else on your hard drive, or written down. You can easily copy and paste if you need to do so. If you are concerned about someone else looking at that file, you can compress it using a password.