'Mozilla announced today plans to remove all Firefox legacy add-ons from the official Mozilla add-ons portal in early October. From a report:

The move comes after Mozilla updated the Firefox core to use a new add-ons system based on the Chrome-compatible WebExtensions API. This new add-ons API replaced Firefox's old XUL-based add-ons API in November 2017, with the release of Firefox 57. All Firefox legacy add-ons stopped working in Firefox 57, but Mozilla continued to support them in the Firefox Extended Support Release (ESR) 52 branch. Support for Firefox ESR 52 will end on September 5, in two weeks, meaning there won't be any official Firefox version that supports legacy add-ons anymore.'

-- source: https://tech.slashdot.org/story/18/08/21/196225

Cheers, Peter