You can kill the process firefox.exe with Task Manager
(or the XP pro taskill command).
This often happens because firefox is not closing down properly for some reason. (or the profile.ini file does not match profiles correctly)

• see firefox hangs <--- clickable link ---
• "Firefox is already running but is not responding" error - How to fix

If you read the articles you may get clues anda solution to what causes your problem, but at least using Task Manager is better than shutting down the computer.

Thank you. I will try this the next time I have the problem

On XP, with Firefox 3.6.17 or 4.0.1, I see frequent loss in connectivity (unrecoverable), leaving an unkillable firefox.exe process (by any means short of reboot). This seems associated with network changes (VPN on/off, switch from LAN to wireless broadband, etc.). It is a very serious issue, since the XP machine often will not even reboot cleanly (but in any case, who wants a browser that requires a frequent system restart?). Please address this, since it basically makes Firefox unusable.

I have an XP and do not personally see frequent losses of connectivity, or an unkillable Firefox process. (I have Firefox 3.6 and 4). But I am currently using only single machines.

If you would like help I suggest you start your own thread, giving more details so that others may try to reproduce the problem.

This is not somewhere for asking for changes or fixes to Firefox. You could use the feedback option.