Hi - a different spin on that question, perhaps…if you close Firefox, do you still notice stuttering? I’ve noticed fairly consistently that everything on my system is just a little bit janky when Firefox is running, for some reason.
That issue in particular hasn’t been an enormous deal for me with my system, but for folks who are tracking FPS carefully, it might stick out more?
It might also be worth double-checking if the Linux 6.12 kernel on Arch is involved somehow - improved performance via some sort of screen-sharing or recording made me think of this thread, which eventually narrowed down to something in that kernel version: Skipping frames with external monitor plugged in - #26 by willard
Hope that helps,