Firefox has blocked WebExtensions from injecting scripts into some of its sites like
addons.mozilla.org, discovery.addons.mozilla.org, testpilot.firefox.com. This restriction is hard-coded into the Firefox and can not be bypassed.
I have heard Google has done the same with Chrome on Google Play.
All WebExtensions that need to inject scripts (for example to get the links, to copy to clipboard etc) will have the same restriction.
I made a notice to tell users that it is nothing to do with the extension and it is Firefox that is blocking the WebExtensions. Firefox is not blocking itself so normal Firefox operations work on above domains. Older addons will also work. Only WebExtension type addons are blocked this way.