I use private browsing as my default and really only open regular tabs when I want to save something for a long time. My private tabs often close on their own at random times when I'm tabbed out with the Firefox app open in the background. If it happened every time I tabbed out or once a day or something I would assume it's a feature, but recently my tabs went about a week without closing and since then they've been closing when I'm tabbed out for 20 minutes. What's happening?