The organization plans to remove the Compact option from the customize menu and migrate users who use Compact to the Normal mode once the change happens. The preference browser.uidensity will remain for the time being, but it is possible that it will get removed at one point in time as well or that the compact mode value won’t change it anymore at the very least.
This is a terrible decision. I obviously use the compact layout everywhere, because not only does it look better and use less space, it also doesn’t have that insanely oversized back button. This change makes absolutely no sense to me, and I can’t wait until we get a hack to bring it back.
I don’t really care. Options like these stink of poor UI design as well as redundant hacks to please people. UI customization is probably still possible via CSS files, though I haven’t checked that since the move away from XUL years ago.
They should revert to vanilla GTK look-and-feel and let theme designers alter the appearance for those who want to screw up their user experience.