Not userfriendly at all, but there is a solution:

  sudo tee <<EOF /etc/apt/preferences.d/firefox-no-snap >/dev/null
  Package: firefox*
  Pin: release o=Ubuntu*
  Pin-Priority: -1
  EOF

  sudo add-apt-repository ppa:mozillateam/ppa
  sudo apt update
  sudo apt install firefox

I'm getting old for this; Canonical is getting Microsoft's manners.