Being very interested in the IPv6 adoption, I like to see if a page is really using IPv6, IPv4, or a mixture.

This extension shows if IPv4 or IPv6 is used just by looking at the extension icon: the big number is for the server on the address bar, the small number(s) showing if components of the page were served by IPv4 and/or IPv6 servers. (Some IPv6 pages are driven by components on IPv4-only servers. For example, my credit union has a dual-stack main page, but all its online banking is IPv4-only.)

Click on the IPvFoo icon, and every server is listed, and whether it is secured, whether it's cached, and (usually) the IP address. It's all presented in a very nice table.

The only shortcoming I have seen so far is if some components are cached from activity on another tab so a server isn't accessed by the current tab, the IP address of that server wont' show; but a "refresh ignoring the cache" (Ctrl+F5) fixes that, at least for the current page. (I hope a future version would resolve the IP address in this situation, but I think it's 5-star-worthy even as it is today.)

Overall, I am very glad to see IPvFoo here for Firefox 57+. I first came across it when I was using Chrome and in Chrome I have absolutely no reservations in recommending IPvFoo!