Firefox now better protects users from “browser fingerprinting” thanks to a technique called “letterboxing” in Tor Browser.
Even if a user blocks cookies from the websites they visit, they can still be tracked through browser fingerprinting.Browser fingerprinting uses the unique combination of a computer's screen resolution, browser and operating system version, installed fonts, and other data to track users' movements.
To combat this, Tor Browser uses “letterboxing” which presents the browser size rounded up to 100px, meaning that in most cases, the browser is the same size as other users. Then, once the page loads and the browser has stopped interacting with the server, the content resizes to the normal window dimensions.
Tor Browser has included this feature since 2015, but it is now integrated into Firefox and is part of the “Tor Uplift” which is gradually moving Tor Browser’s privacy features to Firefox.
Firefox has already implemented protection techniques adopted from Tor, such as creating a "blacklist" of domains that use fingerprinting, font protection, separating cookies by domain, and deploying HTML5 copy countermeasures.
When the user enlarges the window, the largest size it can reach is a multiple of 200 x 100 and the space around it is covered by a gray color. While, when the window is fullscreen, the space around it is black. In addition, there is the possibility of extra zoom, with which the window is enlarged while the space around it is reduced. In the latter case, the window has a space at the top and bottom of the screen which is called a “letterbox” or respectively “pillbox” when the space is on the right and left of the screen. Tor Browser warns when users enlarge their window, a feature that Firefox does not have.
