The User Agent Switcher extension is no longer actively maintained and is not compatible with the new extension framework in Firefox 57+.
User Agent Switcher

The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of a browser. The extension is available for Firefox and will run on any platform that this browser supports including Windows, macOS and Linux.

User Agent Switcher for Firefox

Version 0.7.3 (January 4th, 2011)

Release notes

Supported by BrowserStack.