Web Developer Toolbar and Firebug are two must-haves for the Web developers, I will introduce another powerful debugger, venkman later.
Web Developer Toolbar
This all-in-one toolbar gears towards getting the Web designer out of the tedious work, such as fine-tuning CSS, troubleshooting block layout, validating to the Web standards. My favorite functionality is Display Element Information, the Ctrl-Shift-f shortcut will call a yellow box, once the element is clicked, its attributes are shown in the yellow box, quite nifty.
The console supports one-line command, when you click the Up button on the right, a multi-line textbox is popped out for multiply line code editing. What a considerate design!