An improvement in Firefox 43 made support for legacy-encoded Traditional Chinese pages (Big5, including HKSCS) more compatible with other browsers and with the fonts and input methods of Windows Vista and later, Mac OS X and Linux systems. Users who are still on Hong Kong-localized XP should install a patch from Microsoft.
Firefox users who run Windows XP, read content that might include Hong Kong supplementary characters, and are unable to upgrade from Windows XP, should install Windows XP Font Pack for ISO 10646:2003 + Amendment 1 Traditional Chinese Support.
