I recently had a problem with Firefox and of course the recommended first action is to clear your browsing data which can be quite inconvenient. So if you need to troubleshoot, or want to try something in Firefox before clearing your browser try this first.
Start Firefox using the command line
firefox -P -no-remote
This starts Firefox and allows you you create a new profile, which is essentially like starting Firefox for the very first time. A new profile will have no bookmarks, plugins, or extensions which can then be eliminated (or confirmed) as the cause of any problems.