letterboxing and new window size

Letterboxing is an essential part of browser fingerprinting protection with low impact on usability. Since LibreWolf cares about anti-fingerprinting, privacy.resistFingerprinting.letterboxing should be set to true by default. Not activating it by default puts users who care about fingerprinting in a difficult situation, since activating it will then make you stand out from other LibreWolf users, but not activating it, is also less-than-ideal.