Mozilla can contribute to Firefox 57's success by finishing all the missing WebExtensions APIs. So many extensions are going to be impossible to recreate.
Listen, I don't care whether features that I need don't fit the WebExtensions security model or whether they are confusing to a clueless user. If you can't support these features as part of WebExtensions, please design a separate set of APIs. Fine with me if you market extensions targeting these lower-level APIs as "dangerous, use at your own risk".