I have come across some very sweet firefox extensions that run in the status bar (down on the bottom of the window). Here's a picture of my status bar, I'll explain each extension from left to right:

On the far left we have ColorZilla, with it you can grab any color you see on a web page, its basically the Eyedropper tool (as in photoshop) for Firefox. One downside is that it won't pickup a color from inside a flash movie.

The next extension is FoxyTunes, this extension lets you control your music player from firefox. And by music player, I mean pretty much every music player: iTunes (mac or windows), winamp, windows media player, xmms (linux), and the list goes on. FoxyTunes also shows you the artist and song title, you can see that I was listening to John Scofield when the screen shot was taken.

Next in line is the PageRank Status extension. This displays the pagerank of the page your viewing, just like the google toolbar.

Finally on the right I have the ForecastFox extension showing the current weather (it also shows the forecast, but I chopped that off in this example). As you can see its currently snowing (actually its a blizzard out there), and 32° here in central new york.