[Solved] Can’t open Firefox because a copy of Firefox is already open mac

Following steps would resolve your issue of firefox not loading due to error message that a copy of Firefox is already opened.

In OS X Terminal, open terminal and copy paste the following command

$ cd ~/Library/Application\ Support/Firefox/Profiles/


$ ls -l

This will list the profiles you have in firefox.

if you have multiple profiles, list the one with the most recent date


$ cd rand0m.default
$ ls -la


Here rand0m.default has to be replaced by your profile name fetched above.

remove the hidden parentlock file

 
$ rm -v .parentlock


try to start Firefox again and voila it works !