UserJS.org

Spoof as Firefox or Internet Explorer

This attempts to completely mask your identity as either Firefox or Internet Explorer. Not just the userAgent, but all the other properties are changed as well. Simply set the userAgent to whatever you want in ua.ini, and this script will automatically change all the other properties:
badsite.example.com=4
badsite.example.com=5

window.opera is also deleted, so most other User JavaScripts will not run after this script has run. That is why it has a name beginning with zz, in the hopes that it will be loaded last. No promises, you have been warned. This script is adapted from Andrew Gregory’s page.

Warning: this script may have adverse effects on some pages. You may want to build a list of sites that you want to use it on, and use the @include rule within the comment block to make the script only run on those pages:

// @include http://somesite.example.com/*
// @include http://anothersite.example.com/*