How to use user agent switcher on browser?

I've installed user agen switcher on firefox,the plugin only shows up on addons and I imported xml file How woul'd I set the browser/choose the browser to use not getting the part?

It adds a toolbar button and adds a menu under Tools. Either go to the Tools menu or use the toolbar button and choose a user agent from the list. You may have to customize your toolbar if it is not showing up.