A Firefox Extension Can Cancel a Selected Proxy Challenge

Cancel authentication only for a chosen optional resource URL and only when the challenge comes from the proxy.

For a web resource that should be allowed to fail, an existing Firefox extension can cancel its proxy authentication challenge. This ends that request instead of repeatedly asking the user for another password.

browser.webRequest.onAuthRequired.addListener(details => {
  if (details.isProxy) {
    return {cancel: true};
  }
  return {};
}, {urls: ['https://optional.example/*']}, ['blocking']);

The extension needs webRequest, webRequestBlocking and the appropriate host permission. Replace the example with the specific optional resource URL pattern. The filter matches the resource being requested, not simply the proxy server's address.

isProxy distinguishes the intermediary's challenge from a website's own HTTP authentication. Keeping that check avoids canceling the site's login merely because it uses the same event.

The request is canceled, so the resource remains unavailable. This is suitable only when that loss is intentional, such as a nonessential connection causing repeated prompts. Required content still needs an accepted account and proxy policy.

Firefox has special rules for browser system requests. Extensions can supply proxy credentials for some of those with additional permissions, but cannot cancel or modify those system requests. This page-resource example does not promise silent cancellation for every background operation.

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