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/webstore/*

• 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.

• To allow WebExtensions in Firefox to run on these pages(at your own risk), open about:config and modify the following ³ :

  • Set extensions.webextensions.restrictedDomains to be an empty string.⁴
  • Set privacy.resistFingerprinting.block_mozAddonManager to true.⁵ (must be manually created by right clicking and selecting New > Boolean³)

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

[2] code change, bug report

[3] https://www.ghacks.net/2017/10/27/how-to-enable-firefox-webextensions-on-mozilla-websites/

[4] https://bugzilla.mozilla.org/show_bug.cgi?id=1445663#ch-3

[5] https://bugzilla.mozilla.org/show_bug.cgi?id=1310082#c24