Mac – Firefox won’t open

If Firefox won’t open because it says a copy is already running, you will need to remove the hidden file that Firefox creates to tell itself that its open. This file is called “.parentlock”.

To remove this hidden file we will use the Terminal (aka command line). You can launch the Terminal by typing “Terminal” into the Spotlight search (the magnifying glass in the top right hand corner), or by launching it from the Utilities folder which is located in your Applications folder. Once you have a terminal window up type or copy the following command:

find ~/Library/Application\ Support/Firefox/Profiles/ -name ".parentlock" -exec rm -rf {} \;

That should have deleted the .parentlock file and your Firefox should now open.