- Know and understand the security-related implications of making this change
- In the Tor Browser address bar, type in
about:configand press enter
- Search for
- Set the option to
- Restart the browser for changes to take effect
Tor Browser 9.0 is configured to use letterboxing by default. The technique adds white margins to the browser's frame to let users resize the browser window while still prevent fingerprinting using screen dimensions.
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 coming from experiments that the developers of the Tor browser conducted and is part of the Tor Uplift project that introduces improvements in the Tor browser to Firefox (Tor browser is based on Firefox code).
What are grey bars on resized Tor Browser window?
Tor Browser in its default mode is starting with a content window rounded to a multiple of
200px x 100px to prevent
fingerprinting the screen dimensions. The strategy here is to put all users in a couple of buckets to make it harder to single them out. That works so far until users start to resize their windows (e.g. by maximizing them or going into fullscreen mode). Tor Browser ships with a fingerprinting defense for those scenarios as well, which is called Letterboxing, a technique developed by Mozilla and presented in 2019. It works by adding white margins to a browser window so that the window is as close as possible to the desired size while users are still in a couple of screen size buckets that prevent singling them out with the help of screen dimensions.
In simple words, this technique makes groups of users of certain screen sizes and this makes it harder to single out users on basis of screen size, as many users will have same screen size.
But still I didn't understand 100% of this!