We have seen Mozilla Firefox slowly and gradually adding more features to improve online security and privacy. As a part of its larger project called “tor uplift”, the “letterboxing” technique was borrowed from Tor and made a part of Mozilla Firefox for its update due in mid-May 2019.
It’s kind of complicated and not easy to understand, but you should know it’s a good thing for privacy.
What is Letterboxing?
It is the addition of gray spaces to the sides of your browser when you resize it. To many people, this would seem weird but the main reason behind this is to prevent advertisers from tracking your browsing behavior. These advertisers use the combination of your browser screen resolution and operating system version to create a unique ID, with letterboxing they cannot track you.
Letterboxing lets you increase or decrease the browser window (or maximizing/going to full screen) while adding grey spaces around the content you’re viewing.
Here’s how it works (warning, there’s a lot of technical mumbo jumbo):
When the user maximizes the window, the largest possible viewport is used, again a multiple of 200 x 100. Empty gray margins in the chrome part of the window cover the rest of the screen. Similarly, in fullscreen, the viewport is again given dimensions a multiple of 200 x 100, and the chrome areas around it are set to black.
Finally, an extra zoom was applied to the viewport in fullscreen and maximized modes to use as much of the screen as possible and minimize the size of the empty margins. In that case, the window had a “letterbox” (margins at top and bottom only) or “pillbox” (margins at left and right only) appearance. window.devicePixelRatio was always spoofed to 1.0 even when device pixels != CSS pixels.
How Does it Help?
This way yours and a million other people’s browser resolution stays the same and tracking you is not as easy. This is an anti-fingerprinting technique which prevents advertisers from tracking your browsing activity.
Firefox already has other anti-fingerprinting techniques added as well.
However, the missing feature is a warning that tells you what the consequences can be of maximizing your browser. This lets advertisers know what your monitor’s size is.
Based on Mozilla’s spokesperson’s comment, they don’t plan on adding this feature in Firefox anytime soon.
Letterboxing is not turned on by default and needs to be selected from the browser’s privacy settings.