Mozilla development team announced a new privacy protection mechanism that will come with Firefox 52, it aims to prevent websites from fingerprinting users. The technique is widely adopted by advertising companies via hidden scripts delivered with ads that take the list of local fonts and along with other data create a unique fingerprint (ID) for each user. The feature leverages a whitelist of system fonts for each operating system, the browser will not block queries for system fonts but it will provide the same answer for every user making impossible to discriminate them.”]
Source: https://securityaffairs.co/wordpress/54938/digital-id/firefox-52-privacy.html