Which tool-set are you referring too? These are my three favorites these days.
Web Developer
Adds a menu and a toolbar with various web developer tools.
http://chrispederick.com/work/web-developer/firefox/
I Use the Disable, Cookies, Images, Outline, and Resize menus often.
HTML Validator
The number of errors of a HTML page is seen on the form of an icon in the status bar when browsing. The details of the errors are seen when looking the HTML source of the page.
http://users.skynet.be/mgueury/mozilla/
Not only great for catching validation errors and coding mistakes, but often gives short advice on how to write valid code.
Firebug
Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page
https://getfirebug.com/
Critical tool for tracing how you styles cascade, amongst a multitude of other tools