Select a youtube resolution and resize the player.
In theory this should already be handled, but I probably haven't really tested this case. I will try and see if I can reproduce on Firefox.
Good luck. I can confirm that it still happens in Chromium browsers. To elaborate, if you pause the video and leave the tab alone for a few hours, when you try to resume playback, it only plays for a few seconds, then goes black and shows the loading animation, then starts playing again. That's what I mean by reloading the player. At that time, the quality setting returns to auto.
Does its job pretty well. One downside, if you pause a video and come back hours later to resume it, so that YouTube must "reload" the video player, the quality setting switches back to auto. Or at least it did as of the last version; I haven't had the latest version long enough to know whether it still does this.