This might sound tripe but please try Google Chrome. Firefox has all sorts of funkiness, the card counts doesn't actually count the cards you have in the database, it counts the movements of cards in the interface, and that could be incorrect if the browser refreshes too slow.
http://www.konami.jp/topics/2009/0807/index-e.html
Also if you find something bad happening (outside of a match) refreshing usually helps. Lastly many many people reports Firefox not working when playing an actual match.
Sorry, better support for browsers other than Chrome/Safari will be forthcoming.
Edit: Actually in regards to largest, it seems debatable.
http://www.guinnessworldrecords.com/...game/75891.htm
http://www.guinnessworldrecords.com/...game/69956.htm