Mozilla launches Firefox Quantum, the biggest change in it's history

[Slapdown3]

[QUOTE=UntouchedShadow;52899447]Has anyone else's Youtube viewports shrunk with the new Firefox? It used to be 1280x720 but the size shrank to 854x480 and I don't know how to change it back. If I zoom the whole page out it goes back to normal, but then text is too small...[/QUOTE]

I have a similar issue, except when I move the window to my main monitor it changes in size.