Android Firefox ultrawide fullscreen: best fit instead fit screen?

On a 21:9 ultrawide screen phone (moto g 5G plus, Android 10), while playing youtube videos in full screen, Firefox is using "fit screen" presentation, which touches left and right screen borders but cuts upper and lower parts of the video:

Is it possible to make it "best fit", where it touches the upper and lower sides of the screen and leaves black parts in the left and right sides? This last capture is from another browser (jQuarks).

Tried both the latest Firefox 85.1.1 or Firefox Beta 86.0.0-beta.2 and they both use the same "fit screen" indifferently if the site is viewed in mobile or desktop mode.

Unfortunately, it seems to be a bug which is addressed in Videos with different aspect ratio than device are zoomed in and the current workaround which works well is to use ublock-origin addon and add these 2 filters:

m.youtube.com##.html5-video-player.ytp-large-width-mode video.video-stream.html5-main-video:style(height: 100vh !important)
m.youtube.com##.html5-video-player.ytp-large-width-mode .captions-text:style(position: fixed; bottom: 1em)