Firefox has a blacklist of sites that can't be modified by extensions, specifically addons.mozilla.org and testpilot.firefox.com (which can both install extensi

Firefox has a blacklist of sites that can't be modified by extensions, specifically addons.mozilla.org and testpilot.firefox.com (which can both install extensions). It's there to protect against malicious extensions that might try to install further extensions or otherwise escalate permissions by making use of those sites' permissions. That block is also going to keep uMatrix from being able to block anything specifically on addons.mozilla.org