Firefox not responding problem
When I try to start Firefox, I get a message saying "Firefox is already running but is not responding." Firefox isn't running - how can I fix this?
In order to not accidentally have more than one Firefox running at a time, Firefox creates a "lock" file in your profile directory. Ordinarily, this file is deleted when you quit Firefox, but if Firefox crashes or otherwise shuts down improperly, this file can be left behind, confusing Firefox. We have also seen instances where Firefox doesn't properly delete the file even when it is shut down properly.
To resolve this, you must remove the lock file manually before starting Firefox. Please see the following Firefox Support page for instructions on finding and removing this file: