You are asked this question because you already have an instance of mozilla running.
After your reboot, open a console and try this:
Code:
ps -aux | less | grep mozilla
If you get entries - find the number that is closest to the left (it will be a four or five digit number - called a process id)
Then, once you have found the number:
Code:
kill <your process id here>
Now launch mozilla - should be good.
If this works, look at your xinit scripts, as well as /home/.kde/Autostart (if you are using KDE) to remove mozilla from startup.