It occurs any Firefox (37.something onward, currently on 38.0, didn't occur with 36) closure for me, be it from general computer restart, browser restart from browser updates, clicking on the x - and only for this site; other sessions survive just fine. 
	Chrome seems to actually store the session fine even with a couple of weeks going by and FF eating the cookies a handful of times in between. (But again, Chrome tends to seize up and crash on some of the addons I need, and I have more than enough windows open at all times, especially on laptop.) 
	Cell (Android, FF) doesn't seem to be eating session cookies. It has an update for FF queued, though, so this may change.