`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.