TB has much less manpower than Firefox, so they're mostly following whatever Firefox does. Firefox is indeed in the process of getting rid of XUL in favour of

TB has much less manpower than Firefox, so they're mostly following whatever Firefox does. Firefox is indeed in the process of getting rid of XUL in favour of web technologies, but AFAIK it's not done yet. And yes, on UNIX platforms the "XUL backend" is gtk.