Every damn time Firefox decides to update, it deletes every single one of my Stylus themes. And yet long standing issues like the Bookmark Tool Bar going blank if you watch video in full screen for too long still exist (10+ years now).

Also, whenever Firefox updates, it royally screws up the session that was active. Instead of opening every window and tab that was open, most of the time only one window gets restored. Sometimes none of the windows get restored because Firefox stupidly opens and then closes again, and then when it opens again, all you get is a page telling you about the stupid feature you don't care about that Firefox updated and ruined everything for.