Okay - this just started today: I have two user profiles for my computer. When I try to use Firefox (on the profile I use the most) I get an error message saying "Firefox is already running" - yet I just restarted the computer, and when I look in "processes" I can't find anything running - how can I close out what I can't find? So, I try the other profile. It's error message is "group policy client service failed logon." WTF? What the hell is a "group policy client service?" I tried switching users, restarting, logging off, every combination. HELP!
^ You could also try doing a system restore, too. That's worked for me in the past for certain programs.
I've seen this before, some time ago. I forget how to fix it. There might be some question of which version(s) you have installed/running. Here's Mozilla's page on the problem. Note that a profile for Firefox is not quite the same as a log-in profile for the computer. This person suggests finding the "lock" file and axing it. Also mentioned is that this problem can arise if the computer shuts down while Firefox is still running, which is the only thing that sounds familiar to me so far.
