I have a recent issue with Firefox, I can not set any other browser as default, the checkbox “Always check if Firefox is your default browser” is selected and greyed out, cannot be unselected.
It happened to me on Fedora 32, I moved to Open Suse TW, and it has happened again, both distros on XFCE, I go to XFCE settings > Preferred appications > web browser > Vivaldi.