There might be many instances when Firefox fails to start. These issues could be due to add-ons or due to issues in profile files in Firefox user profiles. If you are facing issues with opening Firefox, you can try out Profile Cleaner add-on for Firefox. ProfileCleaner allows to automatically delete some problematic files from Firefox user profile when the browser exits, thus preventing errors when it starts again.
What this add-on does is to delete all problematic files from user profiles which are causing start up issues.
Please not that, you need to use this add-on only in case you are facing issues with Firefox start up.