Since Firefox 13Beta You have to make this change to have the Add-On Work :) Rated 5 out of 5 stars

Since Firefox 13Beta You have to make this change to have the Add-On Work :

Open the new tab and type about:config in the url bar, click ok at the "i'll be careful" warning and then copy and paste this browser.newtab.url into the search area.

Then when the preference appears double click on it and in the popup type about:blank click ok and viola a blank new tab is what you will have when you want one or NewTabURL will work again.

Regards