I set this up today on my work laptop with (internal) RTX3060. According to the status indicators on the Adjust Video Image Settings page in the Nvidia control panel, super resolution is working in Chrome v124.x and v125.x but not at all in Firefox v126.0. My eyes tell me the same thing. I was able to play a 480p YT stream in Chrome and it looked surprisingly good on my external 1440p monitor. In FF it looks like ass. I may set up a secondary profile in FF just to make sure I haven't changed some config setting over the years that would prevent it from working right in FF. Will update if I find anything interesting.
Edit: Just tried this again with YouTube in FF v126.0 with a clean profile. It does work, but only when the video is full screen (which makes sense I guess, but the behavior is different from Chrome) and I had to manually set the quality level in the Nvidia control panel. In Chrome the auto setting used level 4 (the highest level), but in FF the auto setting only used level 1.