Firefox says only one "copy" can run at a time
I have 2 user accounts on my Mac, only one of which I really use. For some reason, Firefox now refuses to open in that user, saying that there's already a "copy" of Firefox open and that I can't have multiple copies running. (Firefox works fine when I'm on the other user, but it doesn't have all my bookmarks, passwords, etc... not to mention I can't access the non-Firefox files I want.) How do I close or get rid of the extra copy, so I can use Firefox in the other user account? Quitting the app doesn't do it.