I've recently upgraded to Kubuntu 22.04.I've attempted several times to set Firefox as the default browser in about:preferences without success. I select 'Make default', then 'yes' to 'Allow settings change', but the message, 'Firefox is not your default browser' persists.
I've come across the following ticket about this same issue. If I'm reading it correctly it was fixed four years ago.
https://bugzilla.mozilla.org/show_bug.cgi?id=1444313
