Firefox: Change User Agent String

1. Enter about:config in the URL
2. Right-click -> New -> String
3. Type general.useragent.override
4. Enter the new user agent string

For example:
Mozilla/5.0 (Linux; U; Android 2.1-update1; en-us; Nexus One Build/ERE27) AppleWebkit/530.17 (KHTML, like Gecko) Version/4.0 Mobile Safari/530.17


Extra:
* To check your current user agent, enter about: in the URL
* To revert your changes, right-click on general.useragent.override -> Reset. The key will go away when you refresh the page.


Ref:
Changing User Agent in Firefox