At upgrade/update the file "/usr/share/applications/firefox.desktop" must be modified at the the 1st instance of "Exec=firefox %u" to allow Firefox to become the "Default Browser" The following line is is the patch I add to the 1st instance of "Exec=" Change line "Exec=firefox %u" to "Exec=env --unset=GTK_USE_PORTAL /usr/lib/firefox/firefox %u"
*** This bug has been marked as a duplicate of bug 425287 ***