It's not only WebKit, but also the Javascript engine and to make matters worse, on iOS last time I checked the web view that you could use in apps wasn't using the same Javascript engine that Safari was using, which means that an alternative browser is slower than Safari by default. From what I've been hearing, lately developers can use the same engine as Safari inside those web views, but the functionality is still buggy.
And this matters, because the main features that browsers compete on is speed and new features (in HTML5 and Javascript). This is what made people switch to Chrome, making it gain a whooping 40% (or is it 50%) in market share. This is also what made Firefox gain market share from IExplorer.
And if your browser is slower than the default one, then you cannot compete with that default. If your browser cannot be a platform (for add-ons or apps, like what Firefox is doing on Android), then you cannot compete with the default. Chrome on iOS is only a shadow of what it is on the desktop.
And Firefox on Android is the only one that allows me to use useful add-ons, such as AdBlock Plus, HTTPS Everywhere and LastPass. Do you know how awesome that is? And this doesn't work on iOS (i.e. having add-ons) because of Apple's restrictions.
And yes, I'm even rooting for Gekko (instead of WebKit), because it is Firefox that brought us Asm.js [1], I even bought the Humble Mozilla Bundle [2] and they made everybody optimize for it, including Microsoft. And now they are working on SIMD.js as well [3].
I received an iPhone 6 as a gift 3 weeks ago and I sold it - yes it's nice, but it doesn't run Firefox and apparently Apple kicked VLC out of the app store as well. And personally I couldn't picture myself living with such restrictions so I sold it - now I'm waiting for my Nexus 6 to arrive :-)
[1] http://asmjs.org/
[2] https://blog.mozilla.org/blog/2014/10/14/play-awesome-indie-...
[3] https://hacks.mozilla.org/2014/10/introducing-simd-js/