WebExtensions is far from solid as a whole, since most of its parts are simply missing, unstable or undergoing changes. Example: the part of Bookmarks API that would support tags and descriptions.
In the corresponding bug the developers say that no one knows when the mentioned will be implemented, since “this is the question of resources”. Previous API disabled, current API just doesn’t support required features - is it how you encourage developers to use WebExtensions?
If you would prepare the WebExtensions at least supporting the majority of functionality supported by XUL, before disabling the latter, there wouldl be less problems for both legacy add-on developers porting their extensions an users using those extensions.
When you change API next time, please consider making it really solid and reliable and covering the features already existing in present API - before disabling existing API. No need to repeat such an epic fail again.