Linux :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

rm ~/.mozilla/firefox/*/lock ~/.mozilla/firefox/*/.parentlock

workd for me on RHEL5.  My symptoms were that there were in fact *no* firefox-related executables running on the server, via top, or ps -aef, or anything, so I suspected a lock file, I just had to find the darn thing!
Stupid, when I write an application that uses a lockfile, I build in a way to tell that the logfile is out of date (and likely left laying around after a server reboot).

I found the breadcrumbs (but not this exact info) here:

[SOLVED] Firefox is already running, but is not responding...

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