That's fine, and I get that gnome is a platform. The problem is that they're forcing their platform's choices on
every GTK-3 app, which makes it really annoying for people who use GTK (not gnome) apps but which don't like the Gnome platform.
The problem is that GTK was used as a generic toolkit, and they've added a bunch of weird customization's to make it work better with their platform. That fucked over a lot of people who used that toolkit on non-Gnome platforms, and for non-Gnome uses.
Most people using GTK didn't sign up to be part of the gnome platform. Firefox isn't a gnome app. They (and everyone else) assumed it was being developed as a generic and cross-platform gui toolkit. Bit of a bait and switch there.
The tight integration between gnome (the platform) and GTK (the cross-platform GUI toolkit) seems an awful lot like a betrayal, like taking something that was for everyone and well.. taking it. Making it all about themselves, and burning the commons so other people can't use it. Or at least like a heavy-handed attempt to force people to write Gnome apps instead of GTK apps.