I think the opposite is more likely: Firefox and Chrome using the same extension mechanism -- and, inevitably, that means two-way chasing on implementing APIs that prove useful -- means that even if the ability to do more complex apps in Firefox falls in the short run, the ability to more complex apps in the browser in general (or, at least, Firefox, Chrome, and their descendants and others who adopt the same framewoek) is going to accelerate over a longer term.