not all process's show up in gnome-system-monitor, try
if their really is a copy of firefox running it will show up then. If nothing shows up another thing to try is:
ps aux | grep firefox
if there is a firefoxbackup folder then try:
if that still doesn't work try going into synaptic and when you uninstall firefox mark it for complete removal then install it again. If it happens again the terminal output before it fails would help so I can figure out why it happened.
rm -R ~/.mozilla/firefox
mv ~/.mozilla/firefoxbackup ~/.mozilla/firefox
The firefox launcher copies your firefox folder to firefoxbackup then copies it to your ramdisk, it removes the firefox folder then symlinks it to the copy in the ramdisk. Then it uses rsync every 5 minutes to sync the backup with the copy in ramdisk.