Because it happens on startup, repeating the process just hangs again.
If you get in this state, you can fix it by throwing away your saved session. To do this, delete (or rename) the file
C:\Documents and Settings\Application Data\Mozilla\Firefox\Profiles\sessionstore.js
and start Firefox again. Then it will come up, you can make your setting in the dialog box, and at least start browsing the internets again.
I haven't yet figured out how to get the saved session back again.