Privileged Pages

Privileged Pages are the webpages that browser-developers consider as an entitled webpage/s where extensions are tasked to not work/have their functionality ceased entirely.

For Chromium¹:

chrome.google.com

• On the aformentioned domain, ALL Chromium/Chrome extensions will cease their functionality.

For Firefox²:

accounts-static.cdn.mozilla.net
accounts.firefox.com
addons.cdn.mozilla.net
addons.mozilla.org
api.accounts.firefox.com
content.cdn.mozilla.net
content.cdn.mozilla.net
discovery.addons.mozilla.org
input.mozilla.org
install.mozilla.org
oauth.accounts.firefox.com
profile.accounts.firefox.com
support.mozilla.org
sync.services.mozilla.com
testpilot.firefox.com

• Just like Chrome, Firefox's WebExtensions will also cease to work on these aformentioned domains.

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[1] stackoverflow.com, chrome_extensions_client.cc, extension_urls.cc

[2] https://hg.mozilla.org/mozilla-central/rev/39e131181d44