There is a Firefox extension called User Agent Switcher which allows you to easy switch to another User Agent of a browser. Web user agents range from Web browsers (versions of Internet Explorer, Chrome, Safari, …) to Search Engine Crawlers (Google, Bing, Yahoo, Ask, …), and Mobile Devices (iPhone, iPad, iPod, Android, Samsung, Nokia, SonyEricsson, …).
After installed the User Agent Switcher extension, there is a menu under the Tools named: Default User Agent.
To switch to another User Agent for the browser, let select a User Agent in the list then refresh your browser.
If you want to add a new User Agent which is not exited on the list, go to Tools / Default User Agent / Edit User Agents …
On the User Agent Switch Options box, select New / New User Agent
A New User Agent come up with all user agent values are display. Simply add a description and edit any of values to create a new user agent. Then click OK button.
Bonus: there is an Import button in the User Agent Switch Options box to Import User Agents from an .xml file. I uploaded a useragentswitcher.xml with lot of User Agents For Mobile Devices, feel free to download it.