Once again, Mozilla has taken a leaf out of Tor browser’s handbook with the introduction of user anti-fingerprinting technique in Firefox 67 which is scheduled for a release this year in May.
Dubbed ‘Letterboxing,’ this method protects against window-size related fingerprinting which is used for profiling and tracking users.
For the uninitiated, browser fingerprinting means collection of information on computing devices for the purpose of identification.
These fingerprints can be used by advertising networks to fully or partially identify individual users or devices, and it works even when the cookies are turned off.
Letterboxing adds gray spaces to the sides of the web page whenever a user resizes the browser window and removes after the resize operation is over.
It will mask the window’s actual dimensions by adding space to width and height at multiples of 200px and 100px while resizing. So the gray space will be added at the top, bottom, left, or right of the current web page.
Since advertising codes listen to window resize events to gather this info, Firefox 67 will provide a generic dimension to such trackers and bring back the window to its actual size milliseconds later.
So basically, letterboxing delays the loading of actual page content on the newly-resized window; long enough to trick the tracker codes or scripts to read the incorrect window dimensions.
Letterboxing isn’t a new technique since it was originally developed by Tor Browser in 2015. This feature is currently available in Firefox Nightly for those who’d like to use it.
However, the feature isn’t enabled by default, so users will first have to turn it from the about:config page where they need to find “privacy.resistFingerprinting” through the search box and then set the anti-fingerprinting features to “true.”
