Oh for the day when people can get "it's"/"its" right.
No, your premise is wrong. Unless you are proposing that XUL renders all the RISC OS tools itself, there is no way to make it do this. XUL has no knowledge of RISC OS menus, scrollbars, etc. Now. I haven't actually studied XUL in great detail, but my understanding is that it certainly knows about widgets, but ultimately the widget rendering is down to the toolkit - probably GTK.
So, once again, there are only two ways to make it more RISC OS like - either do a full RISC OS front end for Firefox, or better, implement RISC OS functionality in a modified X11 GTK2 backend, which would be beneficial to all apps using GTK2.