Sorry for the delay Ed. My email server went down this morning, and I never got the tracker email for this discussion.
Did you have a look at the information Carol linked to in the links below? If so, anything there of any use?
It's strange that you get the message that Firefox is already running. Can I ask how you know it isn't? It may not show in the Task Manager under the Applications tab, but there may still be a Firefox process running under the Processes tab. (Right click the Taskbar, select Task Manager).
Do you have Firefox set to load every time you start the computer? If so, de-select that, and reboot the system, then see if Firefox works OK.
Next option. Do you have another Admin capable account on this computer? If so, can you log into that, (restart the computer, select the other Admin account to log into, rather than switch users). If Firefox is installed in that account, see if it works. If it isn't install it, and see if it will work.
I'm just wondering if there isn't some sort of account corruption on the account you normally use.
Let us know how you get on.