Start here:

https://addons.mozilla.org/

Extensions are programs that change the browser and its capabilities. IE Tab was mentioned. AdBlock Plus (when combined with something like DutchBlock) will block most ads. ForecastFox Enhanced adds a nifty miniature weather forcast to the bottom of the window. There's all sorts of stuff, and, unlike with IE... it's all free!

Themes are browser skins. They don't add or change functions. They just change the look of the window. Colors, buttons, etc.

Search Engines let you add more options to the search box.

Plugins are 3rd party apps like Quicktime and Flash Player.

So... plenty of things to explore. Have fun!

When in doubt, think about penguins. It probably won't help, but at least it'll be fun.