Greasemonkey - Customize the way a web page displays or behaves, by using small bits of JavaScript.
Random User-Agent - Automatically change the user agent after specified period of time to a randomly selected one, thus...
Site Connector - Reuse data from one website on another.
Tampermonkey - Greasemonkey compatible script manager.
Launchlet - Customize the web with JavaScript or CSS
Chameleon WebExtension - Chameleon is a Firefox extension to Spoof your browser profile.