And here I was wondering where these two hours of battery time went.
I tried out a tool for scheduled shutdowns on my Ubuntu machine yesterday. Firefox was still running in the background, and after the shutdown it turns out that my whole profile (bookmarks, add-ons, etc.) was lost. To make matters worse, Firefox would randomly crash at every second click, be it on the toolbar or on a link. Removing and reinstalling the packages didn’t help, but fortunately i found a solution here. So if this happens to you, all you have to do is a) duplicating the Firefox profile folder and b) editing the path to it in the profile.ini – see the first answer for a detailed description.