[ale] How could I have avoided this?

[John Mills]

ALErs -

Today after my X11 session was murdered by an app, I couldn't restart 
Firefox because somewhere there was a marker that the old Firefox instance 
was still running, so the new one wouldn't start. Eventually I gave up and 
just blew away my '~/.mozilla/firefox' directory, taking with it my 
bookmarks, etc.

How could I just have removed whatever tag was preventing Firefox from 
starting a new session?

TIA.
 - Mills