User Agent Switcher is a pretty useful addon for Firefox. It can be used to switch the user agent string sent in the HTTP GET request to any custom string you want.
Most popular websites display a different UI for iPhone or Android user strings. So, I typically use this addon to switch to the iPhone user string to try this minimal UI.
This addon adds an entry in the Tools menu. Since Firefox hides most toolbar menus by default now, accessing this menu is a pain. Instead, I prefer to use the button that is provided by this addon. This button can be added to the addon bar, from where it is convenient to use.
Tried with: User Agent Switcher 0.7.3