Ok, maybe it's not really true: FireBug and the Web Developer Toolbar might be better add-ons, but this one is really cool.
Flagfox (click here for its Mozilla Add-ons web page) displays a small icon representing the country where the web site you're browsing is located (the plugin traces the IP address). If you click on the icon, you get location details, coordinates, local time and a Google map and satellite view of the area!
After the Display Mail User Agent plugin for Thunderbird, this is the ultimate time-wasting application for Firefox - but it can even be useful. Be sure to install it!!!

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