How to change useragent in Mozilla firefox

1. In the adress bar go to "about:config" (without quotes).
2. Click the button "I'll be careful, i promise!".
3. In the "Filter" box, paste this line : general.useragent.extra.firefox
4. Change the value to any useragent you want. List of useragent : www.useragentstring.com/pages/useragentstring.php