Firefoxs use of GTK is relatively minimal, especially when compared to something like Wireshark. Almost all UI of Fx in done in XUL afaik.
Even though it's minimal on the desktop, the styling of the UI is still affected primarily by the GTK theme. On KDE I'm using oxygen-gtk to make Firefox look native.
This is because the startup times with XUL were simply too slow. The first version of Firefox for Android did, in fact, use XUL just like desktop Firefox. The app startup times were atrocious. The decision to use a native Android UI made the app usable while Gecko was starting in the background.
Sailfish browser isn't XUL UI either though, it's Qt (QML) based but relies on IPC to separate UI from the heavy components: https://wiki.mozilla.org/Embedding/IPCLiteAPI That also provides fast startup.