Youtube player smaller in Firefox 57
The problem is simple. In Firefox 56 and earlier, the youtube player took the majority of screen space (1080p resolution). You couldn't even see comments without scrolling down (another plus). Now with Firefox 57, the player takes a smaller part of the viewport.
I've attached an image of the viewport in Firefox 56. There's no picture for Firefox 57, because I don't want to install it again just to make an image.
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Your screenshot shows an old design that Youtube has replaced with what you've seen on Firefox 57. I can tell that because the "likes bar" and "Add to"/"Share" options now all occupy the same single line. You can read more about the changes here: https://youtube.com/new
It seems that for some reason your Firefox 56 installation still shows you this now outdated design, but it will eventually show the new design too.
I know about youtube's new design, but thankfully, they still allow you to manually revert to the old design. I've done the same thing in Firefox 57, of course. The problem exists anyway.