Useful to diagnose DNS, for example to verify that Firefox complies with your system configuration.
Keep in my that this extension shows the IP that Firefox is currently connecting to; it is not a DNS lookup via a third party.

doesnt work

This works fine, looks fine. But there's one (big) problem: It doesn't show the right IP-address.

Explanation:

Im intersted in the IP-address the browser connected to. This addon displays the IP-address which is available thru DNS. But the browser may be using another address (e.g. one in the hosts-file). These are the situations I want this information for: When I'm not shure which IP-adress is used by the browser. These situations where there's ambuguity. That's when I want to know what IP-address is used by the browser - and that's what this addon doesn't show.

So this addon is useless - at least for me.

of course: YMMV