I could understand if, say, Mozilla decided they wanted to help other open source organisations that were only tenuously related to them in this way because they can afford to, but the GNOME Foundation is simply too small to take on this kind of big project.
Yes, they have no relation to GNOME.
"A significant subset of GNOME users" running Firefox does not constitute a "relation to the development of GNOME". Neither does Linux being the "OS kernel that most GNOME installs run on".
There's not much interroperability (or need for such) between them. The Gnome team develops GNOME, the kernel team develops Linux and the Mozilla team develops Firefox, and that is that. At best, the Mozilla folks tried at times to adapt to the default GNOME themes etc. Not much besides that.
Now, Gnome and Gtk, that's a real development relation. Or Linux (kernel) and libc.
Not an uncommon problem in non-profits/voluntary organizations.