With the release of Zorin OS 17, Mozilla opted to only offer Firefox as a Flatpak from Flathub.
The ZorinGroup contacted the Mozilla devs to work this out and was informed that Mozilla has been working on an APT PPA for Mozilla and Firefox but with an uncertain release date for it and better security and performance for Zorin OS users by adhering to the Mozilla standing flatpak, the ZorinGroup set Firefox as default flatpak in Zorin OS 17.
Waiting for the Mozilla repo would have delayed the release of Zorin OS 17.
Firefox is currently the only non-standard package format in Zorin OS.
In the time since... Mozilla has released the PPA: