“Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system.”
This issue is caused by corrupted sqlite databases, used by firefox for history and bookmark storage. It often occurs when firefox is terminated abruptly while writing to one of these files (e.g. while a new page is loading).
To fix this problem, open a terminal and do the following:
If your bookmarks menu is “greyed out” after running these commands and starting firefox again, close firefox and run these additional commands:
After removing these files and restarting firefox, the sqlite databases will be recreated, attempting to restore your bookmarks and history from backups that are periodically made in the background. If firefox has made a backup for you since your bookmarks were created, you should see everything restored to the most recent checkpoint.