Mozilla will stop offering legacy Firefox extensions as they are soon also no longer supported by the browser. At the end of 2017, Mozilla launched a new Firefox architecture called Quantum that only supports extensions based on the WebExtensions model.
Users who still wanted to use legacy add-ons had to switch to Firefox Extended Support Release (ESR). Firefox ESR is a version of the browser that only receives security updates and is targeted for usage at enterprises and other large organizations, where large updates can be problematic. New features are only added when a totally new ESR version is released.
On the 5th of September this year, Mozilla will end support of Firefox ESR 52, the latest ESR version that supports legacy add-ons. Because there is no Firefox version that supports legacy add-ons anymore from that date, Mozilla has decided to start the process of removing legacy add-ons from addons.mozilla.org.
The removal process starts on the 6th of September. From that date it will no longer be possible to add new legacy add-ons to the Mozilla add-on page. From the beginning of October it will also no longer be possible download legacy add-ons from addons.mozilla.org anymore.
Developers of legacy add-ons can convert their extension to the new WebExtensions model. When a WebExtension version of an extension is uploaded to addons.mozilla.org, users with the legacy version will automatically be upgraded to the new version, according to Mozilla.