How to enable legacy extensions in Firefox

См. https://www.ghacks.net/2017/08/12/how-to-enable-legacy-extensions-in-firefox-57/

To enable legacy extensions in Firefox Nightly, do the following:

1. Load about:config?filter=extensions.legacy.enabled in the Firefox address bar.
2. Double-click on extensions.legacy.enabled to set the preference to true.
3. Restart the Firefox web browser.