Works Very well. (It stoped 2 days ago, following a Firefox update. But after sending Feedback to Firefox and updating, it works again)

Here's the FIX!!
Nothing wrong with Adblock Plus. The problem lies on Firefox.
Today, Mozilla Firefox 66.0.4 has been released and contains a fix for the expired intermediate signing certificate that caused everyone’s addons to be disabled.
So, update your Firefox to version 66.0.4 and addons will work again.

Как вернуть к работе расширения — открываете страницу about:config, находите там параметр xpinstall.signatures.required и меняете на false. После перезапуска браузера всё работает и мы довольны.