Just fine here with Firefox 16.01 Rated 5 out of 5 stars

After having installed protocol 'Microsoft TCP/IP Version 6' from within XP-SP3, even though I remain at this time IPv4 restricted, SixOrNot allows me to discover when a website is "IPv6 ready". Not to mention "information about all the domains contacted in order to load the page" (which is my first motivation). Nice.

This review is for a previous version of the add-on (