Can't open Firefox because a copy of Firefox is already open
Here is what I did to fix the issue:


In OS X Terminal, find your currently used Firefox profile directory (something like):

$ cd ~/Library/Application\ Support/Firefox/Profiles/
$ ls -l

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

$ cd rAnd0m.default
$ ls -la
 
remove the hidden parentlock file

$ rm -v .parentlock

try to start Firefox again


(If the problem was not related to the parentlock file, you will see:
rm: .parentlock: No such file or directory)

If you prefer to use the Finder, just make hidden dot-files visible first, so you can check for the existence of .parentlock and delete it.