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