User-Agent Switcher is a browser extension that can quickly and easily changes your browser’s user-agent. There are 26 popular user-agent strings to choose from. The browser usually send a full information about the operating system and type of browser and this will be possible to modify in a way to change the behavior and preserve user private information.

Some useful user-agents are as following:

1. iOS: an Apple OS for iPhone/iPad devices.
2. Android: a Google OS for various mobile devices including Nexus.
3. Windows Phone: a mobile OS from Microsoft for windows-phone devices.
4. Tizen: a new mobile OS form Samsung.
5. Symbian: an old mobile OS designed by Nokia.
6. Chrome: a browser build by Google.
7. Opera: a browser build by Opera Software ASA.
8. Firefox: a browser build by Mozilla.
9. Safari: a browser build by Apple.
10. Internet Explorer, Edge: two browsers build by Microsoft.

Current supported features:

• Toolbar popup UI provides an easy interface to access 26 user-agents. Clicking on any UA, will change your browser’s user-agent after reload.
• The first row in the panel UI is for mobile user-agents.
• Second row is for desktop user-agents and third row is for desktop operating systems.
• The last row in the panel UI is dedicated for the control options including a button to change the UA to its default value. A button to edit the current UA, you can also use it to copy the current UA to the clipboard. Moreover, you can specify a particular URL or domain so that UA is only applied to that URL or domain.
• There is also a status-bar in the panel to show the current UA and other info as you move the mouse within the popup UI.
• Default UAs can be modified from the toolbar panel UI (v.0.1.4)

You can read more and download this extension over here: https://addons.mozilla.org/en-US/firefox/addon/user-agent-switcher-revived/