I have had the same problem on several occasions. I have no idea why Firefox fails to unload when I shut it down, but you can reload it without rebooting.
Right click the taskbar and select Task Manager. Look for firefox.exe in the list under the Processes tab. Select it and click the End Process button. It will ask for confirmation. When this is done, close the Task Manager and restart FireFox. It should start fine now.
I've noted since installing the most recent Firefox update (1.0.4) that after I close the application I can't start it again. If I shut down (in the process of rebooting) the computer, a message tells me that Firefox needs to shut down.
Apparently, even though I think I have shut down Firefox, the computer thinks it is still running. I've not found a workaround for this . . . the only way I can find to get out of it is to reboot.
Anyone else encountered this? Any thoughts? Workarounds?
I am using a Dell notebook running XP Pro with SP1.