Well XUL was slow on Android too. I'm sure it's XUL's fault. Even all desktop Firefox versions are being slown down by XUL. Thats the main reason why everyone thinks Chrome is fast, because it's UI is fast.

In my opinion they should have gone all-in for QT in mobile and desktop Firefox versions long time ago. You got all major OS targets nowadays in Qt (Win / MacOS / Linux / Android / QNX / MeeGO).