Every time Firefox updates (which is pretty often, these days), it opens up a "What's New" page that, frankly, is a little annoying. Here's how to stop it from doing that next time it updates.
This annoyance is even worse if you regularly switch between different Firefox channels, like the bleeding-edge Aurora channel. To turn off the "What's New" page for good, just type
about:config in your address bar, hit enter, and promise Firefox you'll be careful. Then, search for:
Double click on it to set it to "ignore". Now, whenever you update, Firefox should just open up your home page (or restore your session) as normal, without opening any new tabs that get in your way.