XUL is just themed to look like GTK it's no more native than HTML is.
If whatever’s rendering eventually maps to native API calls I’d consider it native, and I’m pretty sure GTK does native calls. If XUL really did mimic GTK (which I’m pretty sure it doesn’t, a lot of other GTk apps had the same feel as Firefox back when most of its UI was native dialogs) then it’s the best damn mimic job I’ve ever seen
XUL had bindings to supported native widgetsets.
I used to build a Firefox addon and the things you could do were amazing -- I could render into the status bar and popup a native window fed with data from a remote server. All from an XML file and some javascript! And you could style it with CSS!
The hacks and contortions addons have to go through now just to do something like that are insane, and the results are almost always janky as fuck.