What is User Agent?
User Agent is a software agent that is working on behalf of a user for some works like browsing, mail reading on the network. For example, we use Outlook for reading emails is the Mail User Agent. We use Firefox for browsing is the Web User Agent. Generally browsers send requests to network with HTTP headers that contains User Agent string. So changing the Firefox User Agent to other mobile browser agent will work as the mobile browser.
Install User Agent Switcher on Firefox
Go to User Agent add-on website and click on Add to Firefox. After installing, restart your browser once. Then go to Tools menu -> Default User Agent
You can switch through any user agents on anytime and you can add various types of user agents like Opera Mini, Skyfire, Bolt, Nokia s60. Click on Edit User Agents on the menu -> "New" button to add your favorite user agents. For this you have to get User Agent Strings of various browsers.