Basically xpcom and XUL can be treated in same way as gnome-shell.
XPCOM was a object model from the 90s and had several legacy issues. On top of that was XUL and XBL were obselete by improvements to web api and was badly mainatined. Like Gnome-shell, xul extentsion were monkey patching firefox internals. So firefox was limited on certain security and performance improvement. By removing old addons, firefox could start replacing the mess of the old subsystems. XBL was removed for example and xul I think still exist but a fraction of the former self. I don't think the can remove xpcom until they can port a 50% of firefox to rust though
Removing XPCOM is orthogonal to adding rust. In fact, if you search Bugzilla you fill find many references to deCOMtamination, which is about removing unnecessary XPCOM.
