Mozilla Browser FAQ

How do I change the user-agent, so I can access site that check for certain browsers (UA spoofing)?

Special note:
Spoofing the UA is essentially lying to others about what you are using. It is only useful in situations where the website will work in Mozilla, but does not accept Mozilla users. Even in such cases, it is important to let every webmaster know what you are really using, so that he/she can have accurate statistics about how many people use which browser to access his/her site. Wouldn't it be a shame if a webmaster says "I don't even consider whether or not my site renders properly in Mozilla, because none of the people going to my site use Mozilla." When really, there may be quite a lot of people spoofing their UA. Then a user may come along that doesn't know about UA spoofing and isn't allowed to access that site. Be proud to use Mozilla, and don't hide it.

Additionally, if you find a website that does not render properly in Mozilla, read the Mozilla technology evangelism page to find out what you can do. Mozilla is built to comply to W3C standards; so when a page does not render properly, it is almost always the fault of a webmaster, who either does not know about W3C standards, or doesn't care about W3C standards.

The best way to change the user-agent is use an extension, such as the prefbar, uabar, or the User Agent Switcher.
These work by editing the preference "general.useragent.override".