User Agent Overrider
Note: Because Firefox 57 will drop support legacy add-ons, and I don't had any time to migrate it to WebExtensions yet, this project is in maintain mode, bugfix only, no new feature. For Firefox 57+ users, I recommend to use User Agent Switcher instead.
User Agent Overrider is a Firefox extension to override the user-agent string.
Github Release Page, pre-release, unsigned xpi file.
Mozilla Addons Page, formal release, signed xpi file, reviewed by AMO.