With the version 1.0PR of Firefox, two more noteworthy extensions have popped up!

ColorZilla

Sits quietly in the status bar, and offers an "eye dropper" style colour picker, with the option to copy the selected colour to the clipboard ready to be pasted into a stylesheet. It also offers a better zoom function. Facilitates re-creating a site look and feel!

QuickNote

After using Firefox for a while, it's hard to imagine still how "normal people" surf the web. By now, I tweaked my Firefox behaviour with several extensions.
Webdeveloper toolbar
Essential for developing web pages! The toolbar offers too many options to list all, but to name a few that I would not live without anymore:

• CSS tools: edit the style sheet of a page on the spot, great for testing and debugging. Mark links as visited or unvisited to test styles here too.