Turns out there’s a “fake” (transitional?) ‘firefox’ .deb:
http://us.archive.ubuntu.com/ubuntu/pool/main/f/firefox/firefox_1snap1-0ubuntu6_amd64.deb
/usr/bin/firefox in that package ends with: exec /snap/bin/firefox "$@".
And ‘debian/firefox.preinst’ basically runs ‘snap install firefox’. QED.