How much does this really matter to the typical user? I’m not using Firefox because it’s JS and rendering engine are significantly better than webkit. Can’t really tell the difference honestly. I’m still getting all the other features of Firefox that actually distinguish it from it’s competitors. The so-called skin is more like the guts from a user value perspective.
The technical reason is that a very powerful system call is blocked on iOS that’s required to build a custom language runtime needed for a browser. This is done to increase security of the device. The trade-off is that the JavaScript engine and renderer must be shared by all browsers on iOS.
But if you can’t tell the difference, does it really matter?