Things I can't live without:

Link Widgets: https://addons.mozilla.org/en-US/firefox/addon/2933/

Basically, it studies the source for links like "next" and "previous," and lets you page forward and backward with alt+pageup/down. Works great on xkcd, New York Times, and many others.

---

Noscript/Flashblock: 'nuff said.

---

Colorzilla: http://www.colorzilla.com/firefox/

Color picker. Combined with Firebug, great for web dev.

---

Leet key: https://addons.mozilla.org/en-US/firefox/addon/770/

Okay, this one I could live without, but the ability to rot13/hex/morse/octal/binary encode text at will is nice.

---

Facepad: https://addons.mozilla.org/en-US/firefox/addon/8442/

I don't really use this one that often, but it's a good example of Firefox's superiority.