Its even worse when you force Firefox to use wayland its icon doesn’t even show.
Edit: Oh since everyone now is confused; I only have the flatpak version of Firefox installed yet it doesn’t use the pinned icon and doesn’t even use the firefox icon under wayland at all.
I’m using KDE + Firefox Flatpak + Papirus Icons and I haven’t had this issue (so far). Could it be an icon pack issue or something similar? Otherwise yeah it’s either KDE or the flatpak