But also note that the "more permissive add-on API" is why FF extensions break between FF versions...
Note, I think there is commitment to allow FF extensions to do more than Chrome extensions can... Ie. sidebar-tabs etc. will still be possible.
Hopefully, the extensions won't break as much... In related news Mozilla is also moving various features into add-ons to reduce bloat, and be able to update features independently of Firefox. I think Hello, Pocket are great examples of things that will be moved to add-ons. (btw, at the announcement where I heard about some features moving into add-ons the subject got a standing applause!)
So I'm pretty sure we can expect FF to embrace/extend Chrome extension API.