`privacy.resistFingerprinting.letterboxing` is currently a separate setting since it's still rough around the edges. Not sure if it works on Release, I have it set on Nightly.
Set the "privacy.resistFingerprinting" pref to "true"
I had that setting set to true since before the update.
Every site that mentions letterboxing claims this is the setting to use to enable it.
However, unless I misunderstand, it does not seem to enable it.
Does anyone know whether they included this feature, and if so how to enable it?
1: https://browserleaks.com/javascript still detects my window size down to the pixel.