On the new clean profile I created in v126.0, I didn't have a gfx.webrender.enabled and gfx.webrender.all was set to false. Changing gfx.webrender.all to true didn't really change the behavior. Nvidia control panel only shows super resolution active when full screen. Watching the same test video as yesterday at the same requested resolution. I did notice that if I set the Quality back to auto, with gfx.webrender.all = true, it picked 2 today instead of 1. 🤷‍♂️

Edit: One DDG search later https://support.mozilla.org/en-US/questions/1445419