Mozilla will stop supporting Firefox Extended Support Release (ESR) 52, the final release that is compatible with legacy add-ons, on September 5, 2018.

As no supported versions of Firefox will be compatible with legacy add-ons after this date, we will start the process of disabling legacy add-on versions on addons.mozilla.org (AMO) in September. On September 6, 2018, submissions for new legacy add-on versions will be disabled. All legacy add-on versions will be disabled in early October, 2018. Once this happens, users will no longer be able to find your extension on AMO.

After legacy add-ons are disabled, developers will still be able to port their extensions to the WebExtensions APIs. Once a new version is submitted to AMO, users who have installed the legacy version will automatically receive the update and the add-on’s listing will appear in the gallery.

For more information about porting legacy extensions to the WebExtensions API is available on MDN. We encourage legacy add-on developers to visit our wiki for more information about upcoming development work and ways to get in touch with our team for help.

Source: Timeline for disabling legacy add-ons on addons.mozilla.org | Mozilla Add-ons Blog