After first install, you should click the triangle on the toolbar button, select a User-Agent in the popup menu, then the extension begin to working.
You also can add your user agent strings, please read the docs.
You should know what User-Agent is, and what side effect after it is changed. Please don't complain or report any bug issue about "it the website is broken".
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.
This extension is host on Github
And the localization host on Transifex, feel free to help.