I should've been more clear, Firefox is just an example, my problem is the mouse pointer being affected by heavy load of any kind on the GPU/UI thread.
As coldblues said, it's probably Mutter not having an independent thread for the mouse pointer, X11's pointer handling is independent from Mutter/KWin it seems.
As coldblues said, it's probably Mutter not having an independent thread for the mouse pointer, X11's pointer handling is independent from Mutter/KWin it seems.
See the demo here to know exactly what I'm talking about: https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1241