"A copy of Firefox is already open. Only one copy of Firefox can be open at a time."

"To remove .parentlock files you can run the following command in the Terminal:

rm -f ~/Library/Application\ Support/Firefox/.parentlock

(It's not in your profile)

However as said before, removing a .parentlock file won't help fix a stuck Firefox process."

"This kills all Firefox processes on Linux:

pgrep firefox | xargs kill -9"

"I always label an old Firefox application as, say FireFox 3.6.17 in the applications folder prior to installing the newer version of Firefox. After that, I install the newer version and rename that application as, say FireFox 12.0.1.

That way, when you see the dialog box saying that another instance of Firefox is already open, pull up the Mac Dock at the bottom of your screen, and you'll see a second Firefox icon in the dock where there wasn't one before. Right-click on the older icon (if you hover the mouse over the icon, you'll see either ""Firefox 3.6.17"" or the newer one) and go through Options and deselect ""Open at Login."" Then force-quit the older application, and that icon will go away. Gone. Somehow, when you ""threw"" or removed the older icon from the dock after upgrading, it didn't change the login status of that icon and kept trying to run both versions of Firefox at login. Problem solved.

When you upgrade and label accordingly, ALWAYS turn off the Login status of the older icon before removing that icon and replacing it with the new one."

"There is a command that combines the pgrep with kill, it's pkill. So:

pkill firefox"

On OS X, if deleting the .parentlock or killing the process does not work, you may need to edit/delete the profiles.ini file. I encountered the problem while making multiple Firefox profiles and resolved the issue be deleting all but the default profile from the profiles.ini (not the profile dir). I think the cause in my case was the use_last_profile setting was enabled and the last used profile no longer existed (profile dir deleted).