After switching user agents I wasn't able to get this add-on working. The menu is over complicated and not clear at all. Also, it's impossible to know weather you have changed or not the user agent because the icon or the menu doesn't tell you at all.

User-Agent Switcher by ntninja is way clearer, and worked at first try. You can know at any moment whether there is user agent switch or not by just looking at the icon. That add-on also works on Firefox Android.