Opera also lost most of their distinctiveness (i.e. features) along the way, laid off the bulk of their developers and sold out to a consortium including Qihoo360 (who, as well as the usual FUD—well-founded or not—about Chinese companies, were simultaneously found to be really bad at being a competent Certificate Authority). There’s plenty of scope for Mozilla to have used WebKit and not done the subsequent parts - Brave’s perhaps as good an example of that as any.

Saying that, roc proposes porting XUL and XPCOM which are largely dead these days (the former by virtue of Firefox switching to WebExtensions). With browser.html for Servo being a thing, building Firefox’s UI in a way that it would work in WebKit is more possible than ever.