Problem: I'm having a problem launching Firefox in Linux. It says I already have a Firefox browser launched and I need to fix that before being able to launch another one.
Solution: In your user space directories you find one of the two following folders:
~/.mozilla/firefox/*.default/ or ~/.mozilla/firefox/*.<your netid>
In one (or both) of these directories you will find a file called .parentlock
You need to delete this file. For some reason it wasn't deleted.