There's a lot more to being a browser -- and being Firefox -- than the web rendering engine. We're able to deliver the kinds of Sync features, private browsing, and so on that we think are important.
(I work on Firefox for Android and iOS, and even _I_ don't really care which rendering engine it uses. I care that I can trust it, that the UX is excellent, and that I have my data.)
That is a good point. To me, however, an important part of the appeal of Firefox has always been a belief that having multiple competing rendering engines is fundamentaly good for the open web -- that it keeps the HTML and CSS standards from growing in too much of an implementation specific direction. I had thought, perhaps without grounds, that this view was shared by Mozilla.
This view is shared by many people at Mozilla (but not all, I expect; it's hard to find anything 1000+ people will agree on) and I believe shared by Mozilla overall. I mean, we're not just continuing to develop the rendering engine we already have (Gecko), we're creating another one as well (Servo)...
(I work on Firefox for Android and iOS, and even _I_ don't really care which rendering engine it uses. I care that I can trust it, that the UX is excellent, and that I have my data.)