I originally wrote IPvFoo for Chrome in 2011, to observe whether websites are served using IPv4 or IPv6:

https://github.com/pmarks-net/ipvfoo/

In 2017, Firefox added initial WebExtensions support, so I dropped the extension on addons.mozilla.org, technically functional but full of bugs, and hoped for the best.

This week, I finally sat down with Firefox and ironed out all the problems:

• Added a separate manifest.json that uses "scripts", as Firefox Manifest V3 doesn't support "service_worker"
• Added a button to grant webRequest permissions, as Firefox Manifest V3 doesn't do this automatically
• Hacked the popup dialog to avoid unnecessary scrollbars
• Added an in-RAM hostname-to-IP cache, as Firefox doesn't remember the IP address of cached requests
• Examined details.documentUrl when handling requests from a Service Worker, as Firefox doesn't use details.initiator

I'm pretty sure that covers everything, so IPvFoo now provides an equivalent user experience on Chrome and Firefox. Enjoy!