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Fusion (Firefox USIng ONions) is a Mozilla project to build the cutting-edge security and privacy features for Firefox users.
Fusion will leverage the technologies of the Tor Project, especially the ones in the Tor Browser and Tor Proxy, to bring more privacy options for users.

Fusion was initiated in 2018. Mozilla and the Tor Project are working closely on this project.


Mozilla and the Tor Project are long-time collaborators. In 2016, we started the Tor Uplift project to take this collaboration to the next level, bringing Firefox and Tor Browser closer together than ever before.

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 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 the Tor Uplift project 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 is disabled by default, but can be enabled by changing a preference value. The Tor Uplift project saves the Tor Browser team work since they can just change preferences instead of updating patches. It also 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.

The primary target of the Tor Uplift project was two features: First Party Isolation and Fingerprinting Resistance. First Party Isolation was shipped in Firefox 52 (off by default); the MVP of Fingerprinting Resistance will be shipped in Firefox 59 (also off by default).

Fusion is the next big step of the Mozilla and Tor collaboration. We hope to move the needle on Web privacy based on the success of the Tor Uplift work.

Project Vision

Mozilla and the Tor Project are aligned with each other on the mission to protect user privacy on the Web.

  • The fourth principle of The Mozilla Manifesto is "Individuals’ security and privacy on the Internet are fundamental and must not be treated as optional."
  • The Tor Project's mission is "to advance human rights and freedoms by creating and deploying free and open anonymity and privacy technologies, supporting their unrestricted availability and use, and furthering their scientific and popular understanding."

We believe the collaboration between Mozilla and Tor can create a positive, significant impact on the world, and help users to regain their privacy.

Project Goals

Ultimately, we hope to integrate full Tor Browser features in Firefox. There are many potential paths to this. For example, enabling some features by default and but other options only in Private Browsing Mode.

We need a lot of research and experiments to support the decision-making for the final solution. However, the clear short and mid-term goals are:

  • We will improve Fingerprinting Resistance by
    • making fingerprinting resistance more user-friendly,
    • minimizing Web breakages caused by fingerprinting resistance, and
    • conducting a browser fingerprinting analysis research project to help us figure out the best defense strategy
  • We will implement a proxy bypass testing framework for Firefox
  • We will determine how best to integrate the Tor proxy into Firefox
  • We will allow First Party Isolation and Fingerprinting Resistance to be enabled only in Private Browsing Mode

Project Lists

Tor Uplift

The Tor Uplift project is aimed at landing all Tor Browser patches so that Tor can directly use Firefox main trunk instead of a fork.

First Party Isolation

The First Party Isolation project is part of the Tor Uplift initiative.
It implements one of the Tor Browser core features (Cross-Origin Identifier Unlinkability).
First Party Isolation (also called "double keying") was incorporated in Firefox 52 with the preference "privacy.firstparty.isolate". It provides a very strong anti-tracking protection by preventing third parties from tracking users across multiple sites.

Fingerprinting Resistance

The Fingerprinting Resistance project is part of the Tor Uplift initiative.
It implements another Tor Browser core feature (Cross-Origin Fingerprinting Unlinkability).
Fingerprinting Resistance (also called "anti-fingerprinting") was incorporated in Firefox 59 with the preference "privacy.resistFingerprinting". It is a defense against browser fingerprinting, which is a widely used Web tracking technology to identify individuals.

Getting Involved

The easiest way to get involved in the Fusion project is to help us writing code, running tests and filing bugs.

If you are interested in contributing to Fusion, drop by:

External Links