I've had a problem with App Tabs before, but having Firefox not delete browsing history after closing solved that rather quickly.
This morning when Firefox updated itself, I had lost my App Tabs which didn't seem like that big of a deal as they tend to disappear every now and then, but usually just re-opening the browser, setting them up again fixes it. Right now when I add App Tabs, close Firefox and start Firefox again, they're gone.
I haven't changed any settings in Firefox prior to or after the update for it to act this way.
Thank you! Showing windows and tabs from last time really did the trick. I do, however, think it's a bit absurd that Firefox team has yet to think of such things with new iterations of Firefox and with their smaller updates.
Help, where is Firefox/Tools > Options, so that I can try the same thing that you suggested above? There is no such thing as tools under my Firefox menu, nor is there an options under my tools menu. I have this same problem that tab groups and app tabs disappear after restarting my Mac. I do not have the 'clear history' setting turned on, so I don't think that is the problem.