FireFox is already running open the TaskManager and kill the process Firefox.exe.
Now you should be able to get it started in a visible screen.
You could also try making use of the handy, dandy, Windows system restore feature and restore to a point prior to the problem.
I am having a problem with my Firefox since I let my son on the computer yesterday. When I try to open it, a window pops up informing me that firefox is already running, but is not responding and that I should close the existing firefox process or restart my system.
The problem is that this just keeps happening when I never opened it to begin with! Right from starting the comp up and even after I restart it. I have no idea how to fix this. Restarting the comp did work before as I can recall this happening in the past but I just can't use my firefox at all now. I have the latest version of firefox too.