The repo is not hidden it does not exist. Firefox is one of the few apps that gets published directly by the devs, and is not built on Flathub’s infrastructure.
Normally, this should link to a page from Mozilla. If you look the Flathub page up in the Wayback Machine with an save from 2023, you can see it links to Mozilla’s Mercurial page.
So this currently linking to GitHub is likely an website bug. Do also note that OBS, which is also build externally, also experiences some issues on the Flathub website currently.