Ah, cursors. This seems to be one area where the Wayland situation is comically bad. I use Sway, and have these in my config:

  output eDP-1 scale 1.5
  seat seat0 xcursor_theme Adwaita 96

Well, I get at least six different cursor sizes depending on the window hovered. Sample apps with rough eyeballed scales: Firefox gets 1×, XWayland gets 1.5×, Alacritty 2×, Sway 2.5×, Zeal 4×, and I can’t remember what the app was that had something else again…

Is it on modern Ubuntu? Because it may be caused by the fact that Firefox is packaged as a snap.

Probably some ubuntu fuckery. I have Firefox in a container as well (but it's Flatpak) on Fedora, and it uses the same cursor as the rest of the system.