The Mozilla Firefox Add-ons site offers half a myriad of extensions with which Firefox can be altered to offer functionality that I consider useful. I don’t use many extensions, but those that I do use I love:

• whatmon helps me keep an eye on my servers
• del.icio.us (finally updated to be Firefox 2.0 compatible) is a neat and clean interface to del.icio.us bookmarks
• QuickProxy allows me to simply click proxy use on or off. I don’t use this often, but when I do, it is a timesaver
• When not at home, I use Sage for reading RSS feeds
• coComment keeps track of where I leave which tracks

whatmon heads the list not only because I wrote it, but because it is most useful to me. ;-)