If Firefox/Iceweasel is “already running, but is not responding”, and you are sure it’s really not running (e.g. after a clean boot), remove its lock with
$ rm -i ~/.mozilla/firefox/*/.parentlock ~/.mozilla/firefox/*/lock
If Firefox/Iceweasel is “already running, but is not responding”, and you are sure it’s really not running (e.g. after a clean boot), remove its lock with
$ rm -i ~/.mozilla/firefox/*/.parentlock ~/.mozilla/firefox/*/lock