After a crash, do you get messages saying firefox is already running and to shut down your old instance before starting up a new one? You need to delete the invisible “.parentlock” file in ~/.mozilla/firefox/$profile. It might also help to kill any gconfd processes and remove any files named ior anywhere under ~/.gconf (find them with find ~/.gconf -name ior).

Why can’t these apps store some kind of host-process identifier in their lockfiles, so they can detect staleness? (See eg. screen, which easily distinguishes local-live, local-dead, and remote.) gconfd is especially annoying, because it doesn’t report the errors except to the syslog, so the user just gets the mysterious symptom of their configuration settings disappearing.