When starting Firefox (version before 1.5), a dialogue for selecting the profile you want to use appears instead of the browser window. If you want to select your normal profile, a message will appear that the profile is currently in use. The programme behaves as if Firefox is already running, but this is not the case.
Firefox was terminated improperly during the last session. This left a lock file in your home directory. This file locks your profile.
Simply delete the lock file. The easiest way to do this is to use the following command, which removes the lock for all profiles at once:
user@linux> rm ~/.mozilla/firefox/*/lock