So you can't innovate on that, at least meaningfully. If you want the browser with best website support, then you'd go with Chrome. If you wanted your bookmarks from your desktop to be in sync with mobile bookmarks, then most likely you are going to install Chrome, because most likely you had Chrome on your desktop (as it is the most common desktop browser).
Also, an apple policy change to lifts the restrictions for non-apple apps to not be extendable, is a different beast entirely from allowing custom web rendering engines.
They have to come from the app store, which greatly reduces the number of plugins available compared to FF, and subjects them to the same strict policies as their apps.
So you can't innovate on that, at least meaningfully. If you want the browser with best website support, then you'd go with Chrome. If you wanted your bookmarks from your desktop to be in sync with mobile bookmarks, then most likely you are going to install Chrome, because most likely you had Chrome on your desktop (as it is the most common desktop browser).
Also, an apple policy change to lifts the restrictions for non-apple apps to not be extendable, is a different beast entirely from allowing custom web rendering engines.