R1:0cd5031ada4e

Add hidpi media query for mozilla browsers

Summary: Modern mozilla browsers use min-resolution.

Test Plan: Regernate sprites, use site in Firefox, see correct header image.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D13555