The WebExtensions technology that use the same extension API supported by Google Chrome and Opera. So the API is also fully compatible with multiprocess Firefox (is called Electrolysis, sometimes abbreviated to e10s). The Electrolysis split Firefox into a UI process and a content process. The splitting UI from content means that when a web page is devouring your computer’s processor, your tabs and buttons and menus won’t lock up too.
If you want to install the Turn Off the Lights Firefox extension and you see this error message “Not available for Firefox 48.0”. Then this are the important notes on what web browser you can use this Turn Off the Lights version.
Version 126.96.36.199 -> that is the last XUL version for Firefox v48
Version 188.8.131.52 -> that use the WebExtensions version for Firefox v49 and higher
You can download both on this page https://addons.mozilla.org/en-US/firefox/addon/turn-off-the-lights/versions/