I default to a portrait orientation in my phone, when entering full-screen on a YouTube video Firefox automatically rotates the screen to landscape and then back again to portrait when exiting full-screen, that's great, however if I then change tabs and return to the tab where the video was playing (which is no longer in full-screen) Firefox rotates the entire site back to landscape mode. 100% reproduction rate. Temporary fix is entering and exiting full-screen again but the bug happens again if I change tabs subsequent times.