I’m going to take a guess that it’s Nvidia drivers and KDE Plasma’s Wayland session not agreeing on something.
Edit: as for Firefox, it’s the Flatpak. It may take a little longer to launch on first-boot, but launching it twice sounds weird. I’m still blaming Nvidia because I cannot reproduce this on my AMD desktop (running the Desktop image) and my Ally X (running Bazzite-Deck).