Looks like a pretty sweet tool for debugging CSS selectors in Firebug.

IE8 Developer Tools and console

Unit testing framework for FireBug/FireFox

Cool extensions for Firebug:

- Code coverage
- Firecookie
- YSlow

The latest "downlevel" firebug uses the pi debugger. I'm just upgrading now...could be a good alternative for Google Chrome debugging as well as IE.

You can get older versions of Firebug here. I recommend NOT using 1.20b6 as it has a serious performance impact on using GMail. Grab 1.20b4 which is a much more stable release and works fine with GMail.

GMail is severely degraded when running Firebug 1.2.0b6 (for Firefox 3). It's much more confusing as to how to disable Firebug completely for particular domains. I'm using Google Apps for domains, and editing a mail message is just about unusable if I have Firebug enabled in the browser.

Firebug-clone for IE (from Microsoft)

Quoted: The Microsoft Internet Explorer Developer Toolbar provides a variety of tools for quickly creating, understanding, and troubleshooting Web pages.

IE8 supports client-side debugging with a feature that looks identical to Firebug - yeah!