I have to agree with Vinni here, particularly as a former Opera user which is a browser that took the polar opposite of this approach and yet remained by far th

I have to agree with Vinni here, particularly as a former Opera user which is a browser that took the polar opposite of this approach and yet remained by far the lightest, most performant of the main browsers up until version 12. It is absolutely possible to have your cake and eat it here, and the main reason for this is that extensibility - from an engineering perspective - is complex. Providing extension APIs necessitates generalised code, that fits a range of use-cases and is parameterised in a generic manner. Providing specific browser features on the other hand requires specific code, which can be written in a targetted fashion for the single feature, and therefore can be made much leaner and optimised. This is absolutely not always the case - you can of course have the worst of both worlds (inflexible non-extensible apps with inefficient badly written code), but I would not underestimate the heaviness of extensibility, nor the potential "lightness" of built-in features. --- That said - I'm impressed with the performance of 57 and the ease of writing WebExtensions, so I'm pro this move personally.