User Agent Switcher
The User Agent Switcher extension is not yet compatible with the new extension framework in Firefox 57+. I do plan to update the extension, but it will require being rewritten from scratch so it is not possible for me to guarantee any release date.
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.
Supported by BrowserStack.