This can happen when your X session shuts down unexpectedly or there's a power loss, etc.
In your ~/.mozilla directory there is a firefox subdirectory and in that subdirectory there's another one
called default.skn.
In that default.skn directory, there is a file called lock that normally gets replaced every time
you open a normal Firefox session. If Firefox doesn't shut down correctly (for whatever reason),
this file will just stay there and next time Firefox tries to run, it thinks it's already running.
You can go into that directory (/home/your_name/.mozilla/firefox/default.skn) and delete the
lock file while Firefox is not running, then Firefox ought to run normally next time.
There are other circumstances that will cause Firefox to think it's already running, including those
times when it really IS already running.
But the above is something that often happens when there's
a system shutdown that isn't "clean."