The User Agent Switcher extension adds a menu inside Tools option of the toolbar menu to switch the user agent of a browser. The extension is available for Firefox and will run on any platform i.e. this browser supports including Windows, OS X, Linux and Mac.  

Please find the link below to download the user agent switcher-
user-agent-switcher

How to use- After changing the user agent, reload the page or open the url in new tab.

 

Source: http://selenium-makeiteasy.blogspot.com