They look the same to me. Theater was centered on the screen. Look how far the elbows are from each edge of the screen.

seeing it on my screen the theater one looks smaller, but maybe not. But it SHOULD be the entire length of the black area, and the black area should be probably another couple inches taller as well. Theater mode should take up the entirety of the black space, until today it always did, so something has changed and I am not sure what....if it is youtube messing up again, or if it is something with firefox that may have been messed up with the windows 10 upgrade

How does Full Screen mode work? Does that behave the same? Did you try other resolution setings?

It could be a restriction of the HTML5 media player that YouTube offers for current Firefox releases.

Full screen mode works perfectly, no problems at all.

Resolution settings in youtube? Yes I have tried all the settings and nothing changes.

I did find a way to make it slightly better, though it is not back to the way it was and so is not what I would consider a 'fix'. I zoomed IN of all things. This made the writing smaller but for some reason allowed the theater mode to get slightly bigger, it's now big enough that it is close to what it used to be. Again, not a fix, but at least close.

Are you sure that you weren't zooming the page previously (View > Zoom > Reset)?

yeah I wasn't zoomed. I can tell by the size of the font. But I am just going to go with it as it is. It isn't like it was before, but it's decent enough and I don't want to fiddle too much with stuff and really mess it up. Hopefully it will be like most problems with youtube and fix itself eventually.

I figured this was not a firefox problem but trying to get support of any kind from youtube is just never gonna happen. Thanks for all the help...if nothing else you helped me to calm down a bit and stop ranting hehehehe.