Tools for Firefox
Mozilla Firefox is the browser of choice for the lion’s share of web developers, and therefore has some of the most extensive and well-respected development tools.
Firebug
The Firebug extension for Firefox is indispensable for jQuery development:
http://www.getfirebug.com/
Some of the features of Firebug are :
An excellent DOM inspector for finding names and selectors for pieces of the document
CSS manipulation tools for finding out why a page looks a certain way and changing it
An interactive JavaScript console
A JavaScript debugger that can watch variables and trace code execution
Web Developer Toolbar
This not only overlaps Firebug in the area of DOM inspection, but also contains tools for common tasks like cookie manipulation, form inspection, and page resizing. You can also use this toolbar to quickly and easily disable JavaScript for a site to ensure that functionality degrades gracefully when the user’s browser is less capable:
http://chrispederick.com/work/web-developer/
Venkman
Venkman is the official JavaScript debugger for the Mozilla project. It provides a troubleshooting environment that is reminiscent of the GDB system for debugging programs that are written in other languages.
http://www.mozilla.org/projects/venkman/
Regular Expressions Tester
Regular expressions for matching strings in JavaScript can be tricky to craft. This extension for Firefox allows easy experimentation with regular expressions using an interface for entering search text:
http://sebastianzartner.ath.cx/new/downloads/RExT/