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Prob. sounds stupid but...
Well, a user agent is basicly what browser you are using for instance.
Internet Explorer would be: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
FireFox would be: Mozilla/5.0 (Windows; U; Windows NT 5.1; en-GB; rv:1.7.12) Gecko/20050919 Firefox/1.0.7
Please note these are from windows machines. A linux useragent may look like this: 3.1-RC1  Mozilla/5.0 (compatible; Konqueror/3.1-rc3; i686 Linux; 20020515) depending on what distro.
I think the one you are talking about is Tamper data.
User Agent Switcher. The extension can be installed here.