The Web Developer extension adds various web developer tools to the browser.
Collector's Note This is the swiss army knife of web developer add-ons and can be used for anything from quickly validating a page to viewing a colored topographical map of DOM elements. Any web developer usually has this installed, and if you don't you should.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters...
Collector's Note This is a more comprehensive view of what is going on in the network layer and I like how it displays the information better than Web Developer or Firebug. It's helpful for debugging what's going on and what is posting at the network/HTTP level of any transaction initiated by your client.
Personas are free, easy-to-install "skins" for Firefox. Personas Plus extends that built-in functionality to give you even more control and easier access to new, popular, and your own favorite Personas.
Collector's Note Personas is a quick and easy way to personalize your Firefox window to say something about yourself. It's a new way to add a twist to the web -- whether it's adding a LiveStrong image or displaying one of your favorite musical artist in your toolbar, Personas livens things up and can help you break away from the dull gray that you're used to seeing around your web pages. You can also select random to have your theme change to ensure a fresh look everytime you use Firefox.
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
Collector's Note YSlow gives your site's front end a report card. It grades your front-end optimization in multiple categories to help you find ways to speed up your user experience for anybody visiting it. This is a very useful tool for checking to see if your site (or other sites) are implementing best practices to ensure a positive and speedy user experience for web users.
Useful tools for the nightly tester.
Collector's Note If you use nightly builds of any Mozilla product this is helpful for quickly identifying your current build ID and tweaking your configuration for testing. It also provides a quick lookup for things like config variables and crash reports that you'd normally have to find by typing about:config or about:crashes. Also provides useful tools for managing multiple profiles. Unless you're running nightly builds you probably don't need this but if you are, it's worth checking out.
Easy shrink any long URL or link in the page to something you can email or Twitter using the TinyUrl service with a single click in your browser.
Collector's Note Quick and dirty way of creating tiny urls for sharing whatever you have open in a given tab. It also copies it to your clipboard for you so it's ready to paste and share.