Indeed the update-alternatives mechanism isn’t integrated with snaps (yet?). This is essentially the same problem as was reported for chromium and addressed by a postinst maintainer script in the transitional debian package that installs the snap. However we don’t have such a transitional package for firefox yet.
Note that the title of this post is a bit misleading: firefox can be set as default browser, using different DE-friendly methods:
- from firefox itself, browse to
about:preferencesand click the “Set default” button - in a GNOME desktop environment, run
gnome-control-center default-appsand select the entry for the firefox snap there - in a terminal, run
xdg-settings set default-web-browser firefox_firefox.desktop