I develop my websites in Firefox (of course) and use 4 extensions heavily. In fact – now I have them, I can’t do without them.
1. Web Developer (https://addons.mozilla.org/en-US/firefox/addon/60)
This is possibly the best tool I have ever seen for website design and development. The ability to turn scripts on or off, disable cookies, examine CSS etc. Where would I be without it?
2. Firebug (https://addons.mozilla.org/en-US/firefox/addon/1843)
I have to admit, that since I installed Firebug, I have used some features in Web Developer less. The best feature is the ability to see how fast your site is downloading – and which scripts/images etc are causing hang-ups.
3. IE View Lite (https://addons.mozilla.org/en-US/firefox/addon/1429)
Because I develop in Firefox, I have to keep checking how it looks in Internet Explorer – because Microsoft still can’t… (I’m sure you know the rest)
4. Html Validator (https://addons.mozilla.org/en-US/firefox/addon/249)
Quick and easy validator to make sure your code is good. Valid Html is not everything, but it’s amazing how much time you can save when it comes to fixing problems when you have good code.
Hopefully you find these extensions useful. It certainly speeds up my design and development.