I have a lot of Firefox Add-ons. They are updated from time to time. Each automatic update opens a "firstrun" page in a new tab. Something like "Firebug Release Notes" or "You have just installed Tab Mix Plus".
How to get rid of them?
In the new Firefox 14, plugins are updated in the background (not only on startup as it was in previous versions). So, it's much more annoying - suppose you are reading some web page, and suddenly a new tab opens!
These "firstrun" pages appear only when you perform updates of the add-ons installed. There is an option where you choose not to update the add-ons automatically but it involves you manually check from time to time if there are any new updates to your add-ons. The thing is that add-ons have minor updates in case a Firefox version doesn't change from something like 14.0.1 to 15.0.0. You should worry when the browser is updated to a major version and then checking for updates is a must because of the main reason: compatibility between browser and its installed add-ons.
I have attached an image where you can see where to disable automatic update of the add-ons. You can go to Tools > Addons > Click the little "wheel" next to Search all add-ons field > Uncheck "Update add-ons automatically".