Did you check and make sure that it isnt running in the backround? Registry
Keys still active? All this stuff can cause it, try installing in Safe Mode.
Also try Opening Command by typing CMD in the search bar at the start menu.
Then Type "Tasklist", see if there are any tasks that look like firefox, or
mozilla realated, the type "Taskkill (two K's) Taskkill /Id (Whatever it is)
hit enter and see. Id try Safemode first, also you can try cleaning your
registry manualy. NOT RECOMMENED FOR NEWBEES. This: REGEDIT the on the top of
the window look for EDIT the FIND type like Mozilla and Firefox, when you
find one delete key, MAKE SURE YOUR ON THE RIGHT KEY, i like to right click
the key to be sure. Delete it, keep hitting EDIT FIND NEXT till there is no
more, do it two times, with a restart in between. Make sure you back it up
also. This would be the means i would use but thats me. Anyway hope these
options might help you. Let me know if it does work.