GNOME do not pay, or ask, Ubuntu to use GNOME. The relationship is the other way around. What reason to use Firefox? The extensions are the first and most obvious reason.
Epiphany's extensions have come a long way; have you checked them out? I'm not saying Epy is for everyone, but it makes much more sense to include it by default and have Firefox in the Main repo.
And GNOME Office is missing a presentation program, among other things. I'd love for them to get more attention but with that glaring hole people will probably use and work on OpenOffice.org. I agree on this point. OpenOffice is still much superior; GNOME's office suite really needs some work. And it would be silly to not include OO.o by default at this stage, since it's a much more hefty download than Firefox, and many (possibly most) people will want it.