I continue to be addicted to Firefox largely because of the following extensions:

Adblock Plus - development of the original adblock stalled a long time ago. I have barely seen a banner since 2001, and wonder how others cope.

FireGestures - Once you learn gestures, everything else is too slow. I've been through several other gesture extensions, and this one is the current favourite, although I do have to switch off a lot of options.

British English Dictionary - It's easy to forget to install your language variant dictionary, as US English is so often the path of least resistance.

Custom Download Manager - I like downloads in a tab not in a separate window.

Undo Closed Tabs Button - A tiny change, but so useful. Other tab options are available.

NoScript - For security and annoyance avoidance, turning off javascript is often a good idea.

Firebug - the last word in web development awesomeness.

Honourable mention to Live HTTP Headers and User Agent Switcher which I install when called for.