“Trident” in the user agent string refer too

There's no supported way to do this in C# without any IE Addons unless the WebBrowser control is running in-proc, in which case you can use the UrlMkSetSessionOption() API. See The User-Agent String: Use and Abuse

If you are willing to use add-ons, see http://www.enhanceie.com/ietoys/uapick.asp

Now, there's an unsupported hack to do this that I wouldn't recommend-- namely, you could replace the COMPATIBLE string with the remainder of the Firefox UA, followed by a CRLF and the text IGNORE:. This would cause the HTTP header to "wrap" into a new header, so you'd be sending Headers that look something like:

User-Agent: Mozilla/5.0 (Windows; U; Windows NT 6.1; en-US; rv:1.9.2.13) Gecko/20101203 Firefox/3.6.13 IGNORE: MSIE 8.0; Windows NT 6.1; Trident/4.0; AskTbMP3R7/5.9.1.14019)

this should do it:

http://en.wikipedia.org/wiki/Mozilla

When users visit a website (via a user agent such as a web browser), a text string is generally sent to identify the user agent to the web server. It is known as the "user agent string". The Netscape web browser identified itself as "Mozilla/" followed by some information about the operating system it was running on.

Because the Netscape browser initially implemented many features not available in other browsers and quickly came to dominate the market, a number of web sites were designed to work, or work fully, only when they detected an appropriate version of Mozilla in the user agent string. Thus, competing browsers began to emulate ("cloak" or "spoof") this string in order to also work with those sites. The earliest example of this is Internet Explorer's use of a user agent string beginning "Mozilla/ (compatible; MSIE ...", in order to receive content intended for Netscape, its main rival at the time of its development. This format of user agent string has since been copied by other user agents, and persisted even after Internet Explorer came to dominate the browser market.

wiki knows everything

also https://superuser.com/questions/113020/why-does-internet-explorer-calls-itself-mozilla