To change the User Agent string, just enter about:config as an address in the address bar of FireFox, the location where you normally enter a URL (link). I recommend to preserve the original value, which you can get when you enter just about:  in the address bar.

Now press the right mouse button to get the context menu and select "String" from the menu entry "New". Enter the preference name "general.useragent.override", without the quotes. Next, enter the new User Agent value you want Mozilla Firefox to use. I added my name and a link to my web site to the original value. You can also pick one from the list of User Agent strings. Check the new value by entering about: in the address bar.


The world
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; SV1; Maxthon)

Firefox
User-Agent: Mozilla/5.0 (Windows; U; Windows NT 5.1; zh-CN; rv:1.8.1) Gecko/20061010 Firefox/2.0

Googlebot/2.1
Googlebot/2.1 (+http://www.googlebot.com/bot.html)