OMG! Thank you SO much! I had no idea an extension as awesome as Stylish even existed!
I had to tweak your code a bit, but I made it work.
The original script did nothing, so I did dome digging.
First stop was my screen aspect ratio. Apparently, 21:9 was not actually it (my resolution is 3440:1440) so I just put that instead of "21/9" for the script to detect it.
Then I removed the "X" from "scaleX", because it was just stretching the video horizontally leaving bars on top and bottom.
"Scale" command, on the other hand, stretched the video on both axes - X and Y.
And the last thing I did is I changed the scale amount to 1.345, because that's the exact zooming that gets you a fullscreen experience with my resolution.
Dude, you f*n rock! Thank you so much for your help!