Just tested this in a freshly installed Kubuntu 22.04 in a VM, and the firefox snap is set as the default browser, and I can change back and forth between it and other applications under System Settings > Applications > Default Applications > Web Browser.
Also verified that it is actually used as the default browser when invoking xdg-open.
Leonora, can you please report whether you're still seeing the problem?
Just tested this in a freshly installed Kubuntu 22.04 in a VM, and the firefox snap is set as the default browser, and I can change back and forth between it and other applications under System Settings > Applications > Default Applications > Web Browser.
Also verified that it is actually used as the default browser when invoking xdg-open.
Leonora, can you please report whether you're still seeing the problem?