Unobtrusive, actually great UI design, does the job.

hiển thị ip tốt

exactly what I was looking for... Thanks!

very helpful for debugging network issues

For those worried about "Access your data for all websites", this is an unavoidable problem with addon permission scopes. Put by the purported (likely?) author on a forum:

> IPvFoo author here. The problem is that there's no way to obtain the (hostname, ip) stream from Chrome/Firefox without requesting the "all websites" permission.

> In theory, browser vendors could define a narrowly-scoped permission that only reports (hostname, ip), or roll this functionality into the browser UI, but neither seems likely to happen.

> I made IPvFoo to promote IPv6 adoption, and wouldn't consider selling it for less than $10M USD. It probably won't ever be worth that much because it's an easily-cloned utility without a "moat", but it's more rational to set a price than refuse to sell under any circumstances.

The addon is open source, and if you insisted, could be built/installed manually. However, I trust it for now from here on AMO.

UPDATE: Thanks for confirming, Paul!

Very practical and useful, thank you!

This should be the recommended extension by firefox. Source code is available at https://github.com/pmarks-net/ipvfoo so I think this can be trusted after review