No Xlib for sure, since it should work on Wayland. I think GTK isn't a good choice for that purpose.
Since Webrender is designed like a game engine, may be it should use SDL or something of that sort.
Since Webrender is designed like a game engine, may be it should use SDL or something of that sort.
It would be easier to simply build a Qt version of the bits that depend on GTK, so Linux users can choose between those two. But considering how little Firefox seems to care about Linux user experience (FF has been broken with transparent GTK themes since forever and nobody cares; FF removed Linux native audio support and nobody cares), I doubt this will happen and I doubt a patch would be accepted upstream for reasons of "maintenance overhead". This ignores that maintenance overhead is much higher when two independent parties have to maintain the upstream and a patchset, but is for some reason accepted practice in the open-source world.
No, they didn't. I'm listening to Soundcloud in my Firefox right now; it's sending sound to PulseAudio just fine.
What you actually meant to say is that Firefox removed support for those 1% of Linux users (i.e. 0.01% of total users) that don't have PulseAudio for whatever reason.