See the [[Install an older version of Firefox]] support article for how to revert to a previous version. As for notifying users for add-ons no longer working, the announcement has been made about a year ago, and several recent Firefox versions have showed notifications about it in the Add-ons Manager, where the soon to be "Legacy" add-ons should be displayed as such, including links for more info, allowing users to switch to other compatible add-ons or watch their migration statuses. Unfortunately, I think many users don’t look there, consequently finding out about the WebExtensions change just now when updating to 57. I’d agree with some form of better warning, but you may understand that not doing so is sort of a trade-off as users may not have liked that either (though it could have been a one-time notification). If you need additional help, let us know. /Edit: For the NetVideoHunter add-on, the [https://mozilla.github.io/extension-finder/ Extension Finder utility] shows the [https://addons.mozilla.org/firefox/addon/1-click-downloader-video-photo/ 1-Click Downloader add-on] to be a good alternative. If you specifically want NetVideoHunter to be compatible, you should contact its developer, but take note of the fact the add-on hasn’t been updated for 2 years, so its development may have stopped already.