To be precise here to avoid Misunderstandings. It is not a Zorin Flatpak. It is the official Flatpak Version from Mozilla.
Here on Zorin 18, You can install Firefox as APT Package when You go in the Software Store and choose the Firefox APT Version. Then You should get an Info Window what explains that the Mozilla-own Repo will be added and then Firefox .deb will be installed from that.
Alternatively, You can add the Repo manually by Yourself.
I personally think, it would avoid some ... Irritations when Zorin would come by default with the added Mozilla Repo. Even when Firefox doesn't come preinstalled. But then it would be simple to install Firefox when prefered and choosing the .deb Version.