These are a great feature; and a perfect way to sidestep the weight (and security issues) of having a ton of tiny single-purpose browser-extensions from authors that you don't know.
However, at the moment, it appears that they don't have a lot of the power of browser-extensions - for instance, I tried to copy the meat of this simple "Popout Tab" extension into a programmable button; but it appears they don't have access to the WebExt APIs:
ReferenceError: Can't find variable: browser
This would obviate any need for a variety of small extensions, especially those that are only occasionally useful, and are only useful on-trigger. They lend themselves well to small snippets of code that don't need updating, and that also makes them relatively secure.