Firefox 57 will support only new add-ons written on top of the newer WebExtensions SDK and add-ons written on the old XUL-based Add-Ons SDK will stop working.
In a blog post last week, Mozilla said it will continue to allow developers to upload and list legacy Firefox add-ons on its add-ons portal (AMO), but they'll only show up for users with older browsers and buried in search results under newer WebExtensions-compatible add-ons.
Only a fifth of all Firefox add-ons are WebExtensions-ready
The release of Firefox 57 is the end of a two year long project that started in August 2015 when Mozilla announced the new WebExtensions API that would eventually replace the older Add-Ons SDK.
The new WebExtensions SDK is also compatible with the universal WebExtensions SDK implemented in Chromium and related browsers such as Brave, Chrome, Opera, and Vivaldi.
In April 2017, the Mozilla Add-ons Portal listed 18,814 add-ons, but only 2,273 add-ons were WebExtensions-compatible. Currently, this number is at 3,633, which is about 19.3 percent of all Firefox add-ons.
On the plus side we have been running Version 57.0b for a couple of weeks and it runs like shit off a shovel [Is that a technical term?Ed.]