Mozilla Firefox will soon support ‘letterboxing’, an anti-fingerprinting technique of the Tor Browser

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Yesterday, ZDNet shared that Mozilla will be adding a new anti-fingerprinting technique called letterboxing to Firefox 67, which is set to release in May this year. Letterboxing is part of the Tor Uplift project that started back in 2016 and is currently available for Firefox Nightly users.

As part of the Tor Uplift project, the team is slowing bringing the privacy-focused features of Tor Browser to Firefox. For instance, Firefox 55 came with support for a Tor Browser feature called First-Party Isolation (FPI). This feature prevented ad trackers from using cookies to track user activity by separating cookies on a per-domain basis.

What is letterboxing and why it is needed?

The dimensions of a browser window can act as a big source of finger-printable data that can be used by advertising networks. These advertising networks can use browser window sizes to create user profiles and track users as they resize their browser and move across new URLs and browser tabs. To maintain online privacy of users, it is important to protect this window dimension data continuously even if users resize or maximize their window or enter fullscreen.

What letterboxing does is that it masks the real dimensions of the browser window while keeping the window width and height dimensions multiples of 200px and 100px during the resize operation. And, then it adds a gray space at the top, bottom, left, or right of the current page. The advertising code tracking the window resize events reads the flawed dimensions and sends it to its server, and only then Firefox removes the gray spaces. This is how the advertising code is tricked into reading the incorrect window dimensions.


Here is a demo of letterboxing showing how exactly it works:

The letterboxing feature is not enabled by default. To enable the feature, you can go to the ‘about:config’ page in the browser, enter “privacy.resistFingerprinting” in the search box, and toggle the browser’s anti-fingerprinting features to “true.”

To know more in detail about letterboxing, check out ZDNet’s website.

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