If you get an error when trying to start Firefox, which complains about there being another Firefox running somewhere, you can solve the problem yourself. The error comes from the existence of two files in your home directory, one of which will tell you where the offending other process is.
Find your Firefox directory
First, visit your own Firefox configuration directory:
% cd ~/.mozilla/firefox
In that directory is an arbitrarily-named directory; yours will be different from anyone else’s.
% ls profiles.ini qm7r1ihw.default
Go into that directory.
% cd qm7r1ihw.default
Find the Firefox lock
You’re looking for a file named lock. It’s actually a deliberately broken symbolic link; the target of that link will tell you where the offending Firefox is running, as well as its process ID.
% ls -l lock lrwxrwxrwx. 1 vtimm01 student 19 Sep 12 12:20 lock -> 22.214.171.124:+5703
If you don’t have a file named lock, skip to “Final Step”.
The host that has the other Firefox is 126.96.36.199. ssh to that host.
% ssh 188.8.131.52
Once you’re logged on, find out if the other Firefox process is really there.
% ps auwx | grep 5703 vtimm01 5703 3.1 3.3 1247244 269248 ? Sl 12:20 0:08 /usr/lib64/firefox/firefox
If there’s no Firefox process, log out of the remote host, and skip to the step “Cleaning Up After A Dead Firefox Process”.
If there is a Firefox process, kill it.
% kill 5703
Make sure it died. You may see your own grep process; ignore that.
% ps auwx | grep 5703
Log out of the remote host.
Cleaning Up After A Dead Firefox Process
If there wasn’t a Firefox process on the remote host, there was at one point, and it died in such a way that it failed to clean up after itself. You can clean up for it. Even if you just killed the Firefox on the remote host, it may not have cleaned up after itself.
% rm lock
Don’t do this until you’ve verified that there’s no other Firefox running. Two instances of Firefox will smash each other’s updates in your Firefox directory, and the only way to clean that up is to delete your Firefox directory and let Firefox make a new one.
Remove the file .parentlock. It’s OK if this file doesn’t exist.
% rm .parentlock
You should now be able to start Firefox.