Presumably because these addons (which you haven't named, so I can look at them) do not run in the webpage at all - hence they can run on non-web pages and pages where addon content scripts are blocked, like the mozilla addons site). This addon requires code to run in the page to capture the users interaction with it (mouse buttons and wheel). If you don't believe me please see the the Mozilla documentation: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Content_scripts