https://spoi.com/software/yto/
Has an option to force video quality, plus a ton of other useful features.
I found it strange how the top lists Chrome, Safari, and Opera. They link to their FAQ about Firefox, but it's like they purposefully didn't give a real answer. I guess being miles from "cool as it needs to be" means... not awesome, but versatile for their needs? It reminds me of how everyone used to speak about Chrome back when it came out and was very incompatible since it was still in heavy development to catch up to all the well-established browsers. I have tons of Firefox-compatible Add-Ons and Userscripts which add features and such to YouTube, so I don't see why it wouldn't be possible. I suppose they just don't like the differences they'd have in coding their thing for it, which I don't understand.
I believe you can make it not adjust the quality based on internet speed in the options.
Actually, you can't. A month or so back, my internet got oddly bad, and it bugged out YouTube. It'd keep lowering its quality to the lowest setting, and I'd have to force it back. I checked the settings, but you can't disable this. In fact, they made this feature worse. A week or two back, I was watching a video, and the quality gradually started going down over the course of 10-15 seconds, which I thought was the video since it wasn't snapping to it. But I checked, and it had gone from 360p to 144p. Shocked the forget out of me that it could slowly go stuff quality and I might not be able to tell since upload/rendering problems are possible, too.
