Default fonts on OS X display some Tibetan characters as whitespace. When used in the addressbar as part of an IDN this can be used for domain name spoofing attacks. Note: This attack only affects OS X operating systems. Other operating systems are unaffected. This vulnerability affects Firefox < 54, Firefox ESR < 52.2, and Thunderbird < 52.2.
https://www.mozilla.org/security/advisories/mfsa2017-17/
https://www.mozilla.org/security/advisories/mfsa2017-16/
https://www.mozilla.org/security/advisories/mfsa2017-15/
https://bugzilla.mozilla.org/show_bug.cgi?id=1360309