Usually when this happens it's because there's a zombie process, which is to say one that didn't shut down properly. Give these solutions a try and if nothing works come back and we'll take another crack at it.
When I try to start firefox, it says that firefox is already running but not responding. Please close firefox before trying to open a new window. I uninstalled the version I had (30.0.1). Then re-installed the latest version (31.0) and the same message came up. Also restored my computer to an earlier time about a week earlier.