Mozilla Firefox 67 could embark on a new anti-fingerprinting technique that protects against specific fingerprinting methods related to the window size. The new technique “letterboxing,” has been recently used to add “gray spaces” to the sides of a web page when the user tries to resize its browser window, which is then removed progressively after the resize operation of the window is complete.

The technique comes from experiments conducted by the developers of the Tor browser and is part of the Tor Uplift project started in July 2016. The goal of this project is to improve Firefox’s privacy protection features by relying on those of Tor. Mozilla will give the next version of its Firefox browser a method to hide the size of the window to advertising agencies. There are indeed dimensions to automatically generate user profiles and follow them during their surfing sessions.

This technique, called Letterboxing, is exploited only when the advertising code tries to repatriate the dimension data.

Mozilla has already committed to limiting the intrusive effects of advertising from Firefox 66, which is scheduled for release on March 19. The Mozilla Foundation intends to block the automatic playback of sounds, not only on PC and Mac but also on mobile.

Letterboxing is currently available in Firefox Nightly and will be soon available in a stable version for all users in mid-May with the release of Firefox 67.