Automatically highlights North American telephone numbers on websites, showing the location (city and state) when you hover over the phone number, based on the area code and exchange.
Here is a little more about the add-on from the developer:
To find out where the phone number is located, you just hover the mouse over the phone number, and in 1/4 second (by default) it will start a lookup of the location of the phone number. As soon as the data is retrieved (typically less than 1/2 second), it will be displayed in an info box by the phone number. The box will automatically disappear when you move the mouse outside of the phone number or info box. You can get full details on the phone number by clicking on the location in the info box.
You can pick up the Phone Number Lookup add-on at the Firefox Add-on website.