Firefox som sikkerhetsverktøy

Plugins:
 * Web developer
 * Firebug
 * Live Http Headers
 * Add n'edit cookies
 * Proxy switcher

You can download the Plugins listed above and even more by just installing the Firefox Security Toolkit (FSTK) which was made by Sven Vetsch (Disenchant):
 * Get the FSTK
 * Some information about it can be found here and here