It would introduce extra work for the ZorinOS developers to maintain package-priority overrides and make sure the package continues to work. Flatpak was a supported and easier to implement way to get Firefox pre-installed. As we have seen above, you have to do extra work to get it to work right as a debian package.
That is unclear, but in theory it's not hard to manually move it. It may also be that because the firefox package is installed, it'll install the Ubuntu 22.04-Version of that package during upgrade, which would potentially install the snap.
We'll see when somebody tries it 