For a web developer, I'd say the must-have firefox plugins (out of many useful ones are):
- Gives you easy access to all kinds of information about the websites you visit. You can see traffic rankings from Alexa and Compete, and you can do many useful searches that explore the site in the big three search engines (Google, Yahoo, Microsoft).
is one of my must-haves. Yslow is from yahoo and extends firebug to show you information that can help you make a site load faster for your users.
The Web Developer Toolbar
was once the must-have plugin to do the live css editing, but in this regard it was surpassed by firebug, but it still has a lot of useful tools for the web developer, the ones I use the most are cookie editing, disabling cache, js, or cookies quickly and quickly viewing all the css or js from the page for debugging.
is a plugin that lets you select the color of anything in your browser, and I use it often to quickly see what color I used earlier or to match colors. Also comes with a color palette that lets you tweak your color selections.