This from my colleague, self-described Security Curmudgeon, Matt DuHarte.
As we were discussing here are some of the Firefox addons that are in my standard load out.
Tamper data - http://tamperdata.mozdev.org/
Hackbar - http://code.google.com/p/hackbar/
Live HTTP headers - http://livehttpheaders.mozdev.org/
Refspoof - http://refspoof.mozdev.org/
User Agent Switcher - http://chrispederick.com/work/user-agent-switcher/
WebDeveloper Toolbar - http://chrispederick.com/work/web-developer/
Firebug - http://getfirebug.com/
Greasemonkey - http://www.greasespot.net/
SQL Inject-ME - http://labs.securitycompass.com/index.php/exploit-me/
Please share if you have some other security related tools you would like to recommend.