There are other zoom extensions, some lets you zoom just a little bit and some let you move the yt video after click zoom BUT this is so simple. You just press one button (Z) and it works. It is no nonsense and it works.

works perfectly, no weird behavior if used on my 16:9 instead of the 21:9

Press Z in youtube in fullscreen in ultrawide monitor. Works like magic just like in chrome.

For chrome this works great. But i cant get it to appear in yt videos in firefox