Can't speak for KDE but if you end up needing a workaround you could try installing and configuring xrdp (despite the name it works with Wayland as well as X) - just make sure to configure it listening on a different port vs KDE's RDP server (or just keep KDE's disabled, I guess).
I'm on GNOME and have both GNOME RDP and xrdp but in practice I usually just use xrdp.
Sightly older version of Firefox here (140.x) on Debian Linux, no issues.
Maybe you have a browser extension that isn't playing nice? Try disabling extensions / maybe a private window & see how it goes.
Or if you're on Windows maybe you have anti-virus/malware software causing weirdness.
EDIT: Could be worth running memtest86+ or similar to rule out any RAM issues, wondering if you're just seeing crashing whenever you use software that starts to use more memory.