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

Solution 1:

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.

Solution 2:

Firefox writes two lockfiles, if they are not deleted during shutdown you have to remove them manually. They are called lock and .parentlock and located in the profile folder.

MozillaZine has an article on how to do that on the Mac.

EDIT: On OS X only .parentlock exists, so it should be enough if you delete that file. The file will have 0 KB size, so you can delete it with no worry.

Solution 3:

This kills all Firefox processes on Linux:

pgrep firefox | xargs kill -9

Solution 4:

Somehow the Firefox process is still running even after you close the program.

First, right click on the Firefox icon and try clicking Force Quit.

If that option is not there, press Option + Apple + Esc. This will bring up a list of running programs. Select Firefox and press Force Quit

If all else fails, type killall -9 firefox into the terminal. This should force the process to close, allowing you to start Firefox again.

Solution 5:

I had this issue on OS X v10.7 (Lion). Nothing showed with ps ax | grep firefox and deleting the lock files did nothing for me either. What did work however was launching Firefox via terminal with the profile manager and selecting a suitable profile:

/Applications/Firefox.app/Contents/MacOS/firefox -ProfileManager

It seems as if my default profile which Firefox was trying to open had been deleted. After selecting a different profile via the profile manager menu I was back up and running.