

You said this happens during Elder Scrolls Online, right? Maybe have her close the game’s launcher while the game is running. In my case, I noticed jittery performance in everything, including moving the cursor, until I closed the launcher. May or may not be the issue, but it does sound similar to what happens to me when I leave that game’s launcher open.
Damn, I really wish I could offer more help beyond ensuring drivers are set up correctly for NVIDIA. I mean, you could also check the power profile too, using something like cpupower-gui to see if it’s in power saving mode. I suppose you could also close other applications to see if one of them is interfering, since you said Firefox may have something to do with it. I suppose it’s possible Snap has something to do with it too.