Swiftcore PHP Development

Web Developer - http://addons.mozilla.org/en-US/firefox/addon/60

I think every web developer would love it, no matter you are a web designer or programmer.

Firebug - http://addons.mozilla.org/en-US/firefox/addon/1843

As a programmer, I need to track the http header and/or AJAX request, and firebug is your right choice.

HTML Validator - https://addons.mozilla.org/en-US/firefox/addon/249

Serial would be your right choice when you are prompted to choose an algorithm among HTML Tidy, SGML Parser and Serial. it would do a SGML analytics first, and then try to use HTML Tidy after that.