Actually not compatible with Firefox Fenix Mobile+m.youtube.com.

Choosing Quality eventually cause Video keep restarting on start. Maybe Youtube try to force it Auto Detect in same time. This issue seem not happen if we use Youtube Desktop site.

The others options just don't work on Youtube Mobile view. Autoplay options also probably won't necessary anymore, as Firefox Fenix has it Built-in by now (though seem not perfect).

Messes up theater mode making videos too tall for the window.

It has only one issue: when playing the next video in a playlist for some reason causes the window to scroll down which leaves the top of the video hidden. This is not a minor issue and that's very unfortunate because this extension would be perfect otherwise.
EDIT 1: Reading back the review several users reported the same issue, some over a year ago.
EDIT 2: Workaround by commenting the scrollTo function in ythd.js and disabling auto-update.

iridium was better.

Auto scrolls down page a bit, cutting off top of video. Very annoying.

Everything else works great.

It keeps scrolling every youtube page and video just a bit so you cant see top of the page or video, supper annoying... I see other ppl have complain about it also, had to delete it for that reason, hope you fix it so i can install it again

Video 2x speed is not enough. Is it possible to add 2.5 , 3x speed?

Not auto pausing when I open a video or playlist item, in mobile or "request desktop' mode.

On android quality works but video speed does not.
Also, "Access data for all websites" is a privacy risk. I understand it is required to enable the extension for embedded videos but since this is not the most common use case for many, an option to disable this permission and only let it access youtube.com would be welcome.

Expanded mode does not fill the viewport, "Full page" overflows the viewport cutting off parts of the video, including playback controls and the scrobbing bar.