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Firefox <3 Tor Browser

by Ethan Tseng and Richard Barnes

If you’ve used Tor, you’ve probably used Tor Browser, and if you’ve used Tor Browser you’ve used Firefox. By lines of code, Tor Browser is mostly Firefox -- there are some modifications and some additions, but around 95% of the code in Tor Browser comes from Firefox. The Firefox and Tor Browser teams have collaborated for a long time, but in 2016, we started to take it to the next level, bringing Firefox and Tor Browser closer together than ever before. With closer collaboration, we’re enabling the Tor Browser team to do their jobs more easily, adding more privacy options for Firefox users, and making both browsers more secure.

The Tor Browser team builds Tor Browser by taking Firefox ESR and applying some patches to it. These changes add valuable privacy features for Tor Browser users, but having these changes also means that every time the Tor Browser team wants to use a new version of Firefox, they have to update the patches to work with the new version. These updates take up a substantial fraction of the effort involved in producing Tor Browser.

In 2016, we started an effort to take the Tor Browser patches and “uplift” them to Firefox. When a patch gets uplifted, we take the change that Tor Browser needs and we add it to Firefox in such a way that it’s disabled by default, but can be enabled by changing a preference value. That saves the Tor Browser team work, since they can just change preferences instead of updating patches. And it gives the Firefox team a way to experiment with the advanced privacy features that Tor Browser team is building, to see if we can bring them to a much wider audience.

Our first major target in the uplift project was a feature called First Party Isolation, which provides a very strong anti-tracking protection (at the risk of breaking some websites). Mozilla formed a dedicated team to take the First Party Isolation features in Tor Browser and implement them in Firefox, using the same technology we used to build the containers feature. The team also developed thorough test and QA processes to make sure that the isolation in Firefox is as strong as what’s in Tor Browser -- and even identified some ways to add even stronger protections. The Mozilla team worked closely with the Tor Browser team, including weekly calls and an in-person meeting in September.

First Party Isolation will be incorporated in Firefox 52, the basis for the next major version of Tor Browser. As a result, the Tor Browser team won’t have to update their First Party Isolation patches for this version. In Firefox, First Party Isolation is disabled by default (because of the compatibility risk), but Firefox users can opt in to using First Party Isolation by going to about:config and setting “privacy.firstparty.isolate” to “true”.

We’re excited to continue this collaboration in 2017. Work will start soon on uplifting a set of patches that prevent various forms of browser fingerprinting. We’ll also be looking at how we can work together on sandboxing, building on the work that Yawning Angel has done for Tor Browser and the Firefox sandboxing features that are scheduled to start shipping in early 2017.

Finally, we should recognize the value of the continued collaboration between Mozilla and the Tor Project with regard to security vulnerabilities. The importance of this collaboration was on display only a few weeks ago, when we were both simultaneously notified of a zero-day exploit targeted at Tor Browser using a vulnerability in Firefox. Working together, we were able to develop, test, and ship a fix to both browsers in under 24 hours.

The collaboration between the Firefox and Tor Browser teams is a great example of how Mozilla’s principles of openness and participation can help advance security and privacy in the Internet. We’re proud of all we’ve accomplished together with the Tor Project in 2016, and we’re looking forward to continuing to making the web more secure and more private.