You may notice that in some pages Adblock Plus doesn't work as expected. When you click the ABP toolbar icon, most of the items that normally appear on the menu are not there.
This happens because "internal" pages in all browsers, including Chrome, Firefox, Edge, and Opera are not accessible to extensions. Internal pages include the New Tab page, the extensions manager, the browser's settings, and so on. Browsers don't allow extensions to inject code into these pages because these pages can change the browser behaviour. Allowing extensions to possibly co-opt the browser would be a security risk.