Rated 5 out of 5 stars

Thank you SO MUCH Adam. This is one of my favourite add-ons and has helped me a lot. IMHO its a "must have" Add-on.

I hope to use it after FF 57. Any plans to port it to webextensions?

This review is for a previous version of the add-on (2.12.0). 

A new WebExtensions version has been submitted to Mozilla for review. After it gets approved there'll be an updated to the XUL version (the one you have) telling you how to switch.