Go to your terminal and run "ps -A"
Mozilla asks for a new profile when it is already running and you try to open up a new instance.
Look for a proccess called mozilla-bin or MozillaFirebird-bin (I think it gets truncated by the ps command to MozillaFirebird)
If it is there run:
killall mozilla-bin
or
killall MozillaFirebird-bin
Then run ps -A again.
If that doesn't work then try the kill command, You need the pid (proccess id number) for the proccesses. Sometimes there will be more then one, due to multithreading or something like that. So do the first one and work your way down.
for example:
kill 3128
then if THAT doesn't work then now we must kill with extreme prejiduce.
kill -9 3128
Look as the ps -A output and see what happens, if it doesn't work try it a couple more times.
Then it should be dead even if it is still there. It will be a zombie proccess and be meaningless.
You should be able to run mozilla now.
THis is the same as hitting ctrl-alt-del in windows and then trying to halt then kill a proccess.
Happens sometimes. ALso work for all other proccesses. (don't forget that you may have to kill all the threads of the proccess if ther is more then one pid for a single program.)