I have two accounts on my iMac. One for me, one for my wife. If I log in, I can open firefox no problem. If my wife logs in (NOT at the same time) Firefox does not open. "A copy of firefox is all ready open" - which it is NOT, as I am NOT logged in and running it. I can not delete the profile from my wifes ~/Library/Application folder as it doesn't exist, because FireFox has never run on her profile.