TL;DR Crashes Firefox = lost tabs -
Using Firefox on Windows 7, the intended feature works well however, it causes Firefox to crash every 1-4 hours. It does not crash on execution, nor depending on the number of tabs open. This bug would be alright if Firefox would then load the previous windows/tabs like it usually does upon reopening... but something about the crash causes the last window you were focused on to be deleted from memory. It doesn't load on reopening (other windows will) and "history" rarelyy has record of it. (Sometimes you can just go history -> closed windows and reopen but typically there are no "closed windows" listed in that list.) I'd give this a lower rating if other "session manager" add-ons were available for Firefox the way they are available for Chrome.