Don't get me wrong, I really liked and wanted to like XUL... XULRunner was awesome when it came out (what a decade+ before electron?). However, everything outside of Mozilla's org that's used it is pretty much stale, dead or unsupported.

Firefox didn't dethrone IE alone, iirc, Firefox didn't even exceed IE usage until after Chrome came out, and even then only because Chrome ate further into IE's share and subsequently passed Firefox as well. Chrome was successful in part because of marketing, but in part because it was such a nicer browser experience. I still don't get why Firefox still has a separate search bar, when the address bar works for searches.

I'm not a Chrome apologist either, I'm frustrated with their decisions to remove browser scripts and make user installed extensions much harder to work with, the confusing nag notice at every startup is irritating to say the very least. I do find the UI much nicer to work with, though I wish it had nearly the advanced controls that FF offers.

That said, most people don't want or need to dig that far into their browser.. they want something to check facebook, watch youtube and send emails.