Here i will list some of the most useful extensions for doing applications audits or pentest using Mozilla Firefox:
* Letting you edit request headers and replay an URL. Look for the Replay button in the live window! Very useful ;)
Web Developer Tools:
* Excellent tool, that let you do a lot of things, a must have. It turns the browser into a complete development Tool.
Switch Proxy Tools:
* This extension allows you to define a list of proxies and switch on the fly with a button, very fast! Tor, Paros, Burp, webscarab, jap.
* Use tamperdata to view and modify HTTP/HTTPS headers and post parameters. Trace and time http response/requests.
* A front end for httrack, you could create website mirrors from the browser.
* Slogger creates a complete log of your browsing history. It can save every page using the same options as the "Save Page As" command as well as saving a customizable plain text history file
Add n Edit Cookies:
*Cookie Editor that allows you add and edit "session" and saved cookies.
If you got another interesting plugin, please share ;)