Normally, it has been fixed in Firefox 30. This one was difficult to find, as it was due to a sophisticated combination: 1. preferences setup to cleanup history

Normally, it has been fixed in Firefox 30. This one was difficult to find, as it was due to a sophisticated combination: 1. preferences setup to cleanup history on shutdown; 2. no new thumbnails created during the session; 3. some specific scheduling of shutdown. I'm one of the devs who fixed it, btw.