We are happy to announce that the Firefox 93 release brings two exciting privacy improvements for users of Strict Tracking Protection and Private Browsing. With a more comprehensive SmartBlock 3.0, we combine a great browsing experience with strong tracker blocking. In addition, our new and enhanced referrer tracking protection prevents sites from colluding to share sensitive user data via HTTP referrers.

SmartBlock 3.0

In Private Browsing and Strict Tracking Protection, Firefox goes to great lengths to protect your web browsing activity from trackers. As part of this, the built-in content blocking will automatically block third-party scripts, images, and other content from being loaded from cross-site tracking companies reported by Disconnect. This type of aggressive blocking could sometimes bring small inconveniences, such as missing images or bad performance. In some rare cases, it could even result in a feature malfunction or an empty page.

To compensate, we developed SmartBlock, a mechanism that will intelligently load local, privacy-preserving alternatives to the blocked resources that behave just enough like the original ones to make sure that the website works properly.

The third iteration of SmartBlock brings vastly improved support for replacing the popular Google Analytics scripts and added support for popular services such as Optimizely, Criteo, Amazon TAM and various Google advertising scripts.

As usual, these replacements are bundled with Firefox and can not track you in any way.

HTTP Referrer Protections

The HTTP Referer [sic] header is a browser signal that reveals to a website which location “referred” the user to that website’s server. It is included in navigations and sub-resource requests a browser makes and is frequently used by websites for analytics, logging, and cache optimization. When sent as part of a top-level navigation, it allows a website to learn which other website the user was visiting before.

This is where things get problematic. If the browser sends the full URL of the previous site, then it may reveal sensitive user data included in the URL. Some sites may want to avoid being mentioned in a referrer header at all.

The Referrer Policy was introduced to address this issue: it allows websites to control the value of the referrer header so that a stronger privacy setting can be established for users. In Firefox 87, we went one step further and decided to set the new default referrer policy to strict-origin-when-cross-origin which will automatically trim the most sensitive parts of the referrer URL when it is shared with another website. As such, it prevents sites from unknowingly leaking private information to trackers.

However, websites can still override the introduced default trimming of the referrer, and hence effectively deactivate this protection and send the full URL anyway. This would invite websites to collude with trackers by choosing a more permissive referrer policy and as such remains a major privacy issue.

With the release of version 93, Firefox will ignore less restrictive referrer policies for cross-site requests, such as ‘no-referrer-when-downgrade’, ‘origin-when-cross-origin’, and ‘unsafe-url’ and hence renders such privacy violations ineffective. In other words, Firefox will always trim the HTTP referrer for cross-site requests, regardless of the website’s settings.

For same-site requests, websites can of course still send the full referrer URL.

Enabling these new Privacy Protections

As a Firefox user who is using Strict Tracking Protection and Private Browsing, you can benefit from the additionally provided privacy protection mechanism as soon as your Firefox auto-updates to Firefox 93. If you aren’t a Firefox user yet, you can download the latest version here to start benefiting from all the ways that Firefox works to protect you when browsing the internet.