Following steps would resolve your issue of firefox not loading due to an 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


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