[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 !