Hence the "scary warnings". I don't think UI manipulation is stupid, I don't believe that Mozilla makes the best UI and extensions can definitely improve on it

Hence the "scary warnings". I don't think UI manipulation is stupid, I don't believe that Mozilla makes the best UI and extensions can definitely improve on it for many more specific use cases. Portability is also nice but not everything needs to be widely portable, I would rather have a nice feature in Firefox than not having it because it wasn't portable across browsers. To be clear I am hugely in favour of WebExtensions. I am glad that they were implemented and I think that they should be the recommended API. But I would love to still have the full-power backdoor for the extensions that are no loner possible. For example I maintained VimFx for about a year after they stopped officially supporting these extensions and it was fine. The biggest pain wasn't actually keeping up with the internal API changes, it was the fact that it wasn't allowed in addons.mozilla.org anymore and there is no good way to distribute self-signed or third-part signed extensions.