Here is some additional information supplied by the ZorinGroup:
Is Firefox installed default as a Flatpak?
We're currently shipping Firefox as a Flatpak as – apart from Snap – it is currently the only Linux package source officially maintained by Mozilla.
Given that the web browser is such a critical component of the OS and it's security is of utmost importance, we want to ensure that the security updates are provided officially by Mozilla.The Firefox PPA that has been mentioned on the forum is not maintained by Mozilla. It is maintained by the confusingly named "Mozilla Team" which is part of Canonical. We wouldn't be confident shipping the PPA by default in Zorin OS as Ubuntu primarily focuses their efforts on the Firefox snap so the PPA could be discontinued at any time.
We have been in touch with Mozilla and they have told us that they are working on providing an official Debian repository. The repository is currently in testing, with only Beta and nightly builds of Firefox available. However, Mozilla have told us that they are expecting to be ready to provide stable builds sometime in January.
As a result, we will be shipping Firefox as a Flatpak in the initial release of Zorin OS 17 but are considering switching to the new official Firefox Debian repo in a point release, once it has been deemed to be fully stable and ready for production use. This change would be subject to our own testing, so we haven't made a final decision on whether we will do this yet.