Greasy Fork is available in English.
Resizes the YouTube player to smaller sizes
I ran it on FF 78.11.0esr / Violentmonkey and it seems to work fine for me. You can try testing it on a fresh copy of Firefox to see if other addons are conflicting with the script.
Thanks for responding. I figured it out: the problem is the windows display scaling! Mine is set to 150%. If I set it back to 100%, it works like charm. I also tried on a clean FF profile, so it is nothing with other extensions.
Just as a reference, if I use https://greasyfork.org/en/scripts/405614-youtube-polymer-engine-fixes with the setting of 'Set player height in default mode 480', it works. However sometimes the 480 size not preferred, so I really would like your solution with the toggle button.
That script has been deleted from Greasy Fork so I am unable to test it for you.
Youtube polymer engine fixes is back...
Your script works for me on 100% and 125% scaling too, but not 150%. However 125% is too small on my laptop (1920x1080).
I was so happy finding this script, unfortunately it doesn't work for me. The icon is there and it stretches the vid info under the player, but the size of the player doesn't change. Please make it work. (FF 78.11.0esr / Violentmonkey)