How to reset Firefox :

A corrupt profile can cause various problems with Firefox.You could create a new profile using Profile Manager.I prefer to reinstall Firefox.

Backup your Bookmarks(.json) to a folder of your choice.You will need it to restore it later.Uninstall Firefox and do not save your settings when asked.After uninstalling Firefox run CCleaner.

Install Firefox.When installing Firefox select do nothing when ask to import from IE.Restore .json backup file you saved to restore your Bookmarks.Configure all your Firefox settings.Install Add-ons and Plugins.

How to reset when using ForceField ( standalone ) or in Browser Security in Extreme :

Turn off ForceField to reset Firefox.After you reset Firefox and everything is running as it should.Close Firefox and Turn ON ForceField and Clear virtual data.

Footnote :

Also turn OFF ForceField for installing and updating Firefox.This is also valid for installing Add-ons and Plugins.

Hope this helps.