I have a couple of extensions that some of you may be interested in:

No Script automatically blocks j-a-v-a-s-c-r-i-p-t (I was redirected when I posted just the word), but allows you to selectively enable it for sites that require it (either temporarily or permanently). This allows you to (for example) enable gardenweb scripts but leave other scripts on its pages blocked (like the ivillage survey; I filled it out once, I'm not doing it again).

Adblock allows you to selectively block advertizements (those not affected by No Script) you find obnoxious. Best part is, the more you do it, the less often you have to do so (there are after all, a finite number of advertizements and most are pretty common).

And just for fun there is Stumble Upon. Just click the Stumble! icon that appears at the top left of the browser window and you will be transported to a random website based on preferences you set up (gardening, science, sports, religion, politics - whatever) that you can rank. You can also rank a website that you particularly like which will lead others with similar preferences to that site.

You can also go to the main Firefox Addon Page and browse through other interesting available addons.

And of course, here is the page for downloading Firefox

Ryan