Firefox addons

FirBug
https://addons.mozilla.org/en-US/firefox/addon/1843 

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page...

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

YSlow
https://addons.mozilla.org/en-US/firefox/addon/5369

YSlow analyzes web pages and tells you why they're slow based on Yahoo's rules for high performance web sites.

YSlow gives you:
* Performance report card
* HTTP/HTML summary
* List of components in the page
* Tools including JSLint