1. Firebug : Firebug needs no introduction, with Firebug you can do lot of stuff like editing a web page and monitoring the page Firebug is the best friend for web developers.
2. Web Developer : With Web developer plugin you got bucket of options to develop/edit a web page, Web developer is developer by Chris Pedrick.
3. Alexa : If you are an SEO then Alexa helps you lot, alexa provides the Website rank.
4. Cookie Manager : With cookie manager you able to edit cookies on the web page, you can also hack some web pages.