The latest version of the popular content blocker uBlock Origin comes with a new protective feature that is exclusive to the Firefox web browser.
The latest version of uBlock Origin for Firefox, version 1.25 or higher, will "CNAME-unlock network requests". Some sites may use a relatively new tracking technique that uses canonical names (CNAMEs) to bypass content filters and ad-blockers.
Basically, what the sites do is disguise an ad-tracking or serving domain by using a first-party domain, e.g. this.example.com. Content blockers have a hard time identifying these redirects, and most use a manual list of known domains. The problem with the manual approach is that it is very easy for sites to change the redirects or even use automated means to switch subdomains.
The latest uBlock Origin version for Firefox looks up non-blocked resources to uncover first-party tracking and block these attempts. The resources are highlighted in blue in the uBlock Origin interface when all connections of the active site are displayed. The uncloaked sites are displayed in a smaller font size underneath the canonical names; these may be used to determine whether a resource should be blocked or allowed. Sites may use CNAME redirects for content delivery purposes.
The resources may be handled just like any other connection using the extension; you may allow some in case they are needed or keep blocking them.
Mozilla's Firefox web browser is the only browser with the required DNS API functionality to make these look-ups. Google Chrome and other Chromium-based browsers don't support this and uBlock Origin cannot look up the requests therefore in those browsers.
Firefox users who have uBlock Origin installed should receive the new version of the extension automatically if automatic extension updates is enabled in the browser. The extension may display a prompt to accept a new permission -- Access IP address and hostname information -- during the update process as it is required for the new functionality.
Additional information about the new feature is available on the project's official GitHub page.
The Firefox version of uBlock Origin is the only version that supports CNAME-uncloaking and thus an effective option to block first-party tracking attempts automatically and not based on manual lists. For now, the Firefox version of uBlock Origin is superior to all other versions of the extension. Whether that is enough for users to switch to Firefox remains to be seen.