Comment 3 for bug 1992688

The favourite icon in the shell isn't part of the firefox package, but the /usr/bin/firefox wrapper script has some logic to rewrite the favourite entry from "firefox.desktop" (deb package) to "firefox_firefox.desktop" (snap). So for dist upgrades, this is important (if the deb is removed, the icon disappears because the shell cannot locate "firefox.desktop", so it hides the favourite entry).

The deb also sets up dpkg alternatives for x-www-browser and gnome-www-browser, which snaps currently cannot do (see https://forum.snapcraft.io/t/firefox-snap-cannot-be-set-as-default-browser/26636).