I have recently refreshed Firefox on my laptop running MacOS. Previously I had changed the full-screen-transition in about:config to be zero, but since refreshing changing the setting hasn't disabled the transition. My config is this currently: full-screen-api.transition-duration.enter 0 0 full-screen-api.transition-duration.leave 0 0 full-screen-api.transition.timeout 0 Despite everything being like before the transition persists... I'm currently on the latest version of Firefox (125.0.3)