If FirefoxOS lies and sends a user agent with "Android" in the title, we get the mobile site. If we remove "Android", we get the desktop site.
Can we encourage you to detect mobile browsers using a more robust algorithm? :-) The recommended method is to use the string "Mobi", which will detect Firefox, Chrome, Android default, Mobile IE and Mobile Opera.
We would very much appreciate this being fixed as soon as possible. At the moment, we may have to send a hacked user agent string to get Wikipedia to work correctly.