Chris Pederick's User Agent Switcher  is an  essential Firefox add-on for anybody in the business of creating mobile  web sites.  This little add-on lets your Firebox (or Flock / Mozilla /  Seamonkey) emulate a mobile device in its requests by sending a  configurable User-Agent string.  I've attached a config file to this  post that contains the correct UA strings for a few common phones and  the W3C Default Delivery Context. This will save you the trouble of  finding the correct strings yourself. You can import this file by going  to Tools | User Agent Switcher | Options | Options | User Agents |  Import...

 
To  make this add-on really useful I highly recommend putting it in the  Firefox toolbar. When you do this you get a little globe button that  lets you very quickly flip between different user agents (as you can see  above). The globe is grey when you are using the default UA string and  changes to blue/green if you have a custom UA string set. To add the  button to your browser, right click the browser toolbar, click  Customize... and then drag the button  called User Agent to some  position in the toolbar.
Link To Download : 
User agent Switcher xml
Link: 
Chris Pederick's User Agent Switcher