Browsers have a list of restricted web pages that do not grant access or permissions to extensions.

These are restricted Chromium¹ domains:

clients.google.com
clients[0-9]+.google.com
sb-ssl.google.com
chrome.google.com/webstore/*
chromewebstore.google.com

These are restricted Firefox² domains:

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

• To allow WebExtensions in Firefox to run on these pages (at your own risk), open about:config and modify the following³:

  • Set extensions.webextensions.restrictedDomains to be an empty string.⁴
  • Set privacy.resistFingerprinting.block_mozAddonManager to true⁵
    • Firefox 60 - Firefox 70: must be manually created by right-clicking and selecting New > Boolean³
    • Firefox 71 and later⁶: paste preference name in search bar > in 2nd column set Boolean from single-choice list > click ➕ button in 3rd column.

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

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

[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

[6] https://www.ghacks.net/2019/11/11/firefox-71-new-aboutconfig-interface-lands/