On November 14 Mozilla will take the biggest gamble in its long history when the organization will ship Firefox 57, the first version of its browser that will stop supporting legacy Firefox add-ons.

This means that starting with Firefox 57 the browser will support only new add-ons written on top of the newer WebExtensions SDK.

All legacy Firefox add-ons written on the old XUL-based Add-Ons SDK will stop working.

Old legacy add-ons will still exist on AMO

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.

Changes are already being rolled out to AMO and Firefox 57 (currently Firefox’s Nightly edition).

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.

Only 20% of all Firefox add-ons are WebExtensions-ready

Firefox’s large collection of legacy add-ons has always been one of Firefox’s strongest points and one of the primary reasons the browser has been popular so many years.

When Mozilla announced the new WebExtensions SDK and the death of the old legacy add-ons system, many feared most add-on developers wouldn’t bother to migrate their add-ons for the newer WebExtensions API and Firefox would lose most of its add-ons, and its strongest attraction point.

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% of all Firefox add-ons.