I just found another FireFox Extension that I love -- WeatherFox. As you can probably guess, it pulls weather information for you, and displays it in Firefox. That would be fine, but this this is infinitely customizable -- placement, information, presentation -- it's really quite amazing. Props to the authors for creating this fine extension.
I was just going to post about Weather fox, but then I also found the StockTicker extension, which puts a nice rotating stock ticker in your status bar, updating and rotating at the intervals you choose. Not quote as custumizable as Weatherfox, but still pretty handy.
Lastly, not an extension, but related to one, is Watchcow.net, which gives you an RSS feed of Amazon items or your Amazon wishlist, so you can see any price changes. They also have a "Feed Injector" extension for Firefox that will automagically generate the feeds for you. But I wanted to use Abilon. So I'm not using the feed injector. So there.