Still works in January 2017 on Firefox 51. The Add-on feature of Firefox seems to have changed how it works in the last 4 years.
In order to use this add-on, I had to use another Firefox add-on "The Addon Bar (Restored)" here: https://addons.mozilla.org/en-US/firefox/addon/the-addon-bar/
Once the " "The Addon Bar (Restored)" was activated I right-clicked on any empty part of the screen and checked "The Add-on Bar". Suddenly a bar appeared at the bottom of Firefox showing my IP Address (from this plugin) in the lower right-hand corner. When I activated my VPN, the "My IP Address" did not update my IP in the display. I had to click on the IP text to get the update.
To get my donation, the developer needs (1) update my IP within a few seconds of my IP changing, and (2) enhance the display such that it shows the country name and/or the flag of the country.
Perhaps clicking on the IP text toggles "IP text only", then "Flag only", then "IP text + country name", then "IP + Text" etc. .
I tested it for two days (48hrs) and noticed that in most cases My Public IP worked flawlessly. Congratulations. How did I do this? I typed in google "what is my ip-adress" or "my ip please". The results I compared scrupulously with the results of this add-on. Far above 90% was a match! This add-on is highly recommended.