These are the tools I use for web development. I mainly use Joomla and am not a hardcore developer, but these tools are absolutely invaluable for Joomla or similar website development.
Collector's Note Anyone who does website development or is just curious on how web pages work needs this plugin. I can't believe it is free.
The Web Developer extension adds various web developer tools to the browser.
Collector's Note This is the second most must-have addon for web development following firebug.
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Collector's Note For use with Firebug. Basically integrates HTML and CSS documentation within Firebug.