To make some internal changes easier, we've now moved the old web interface to http://old.fastmail.fm. For the moment, http://www.fastmail.fm/old/ will continue to work, but this will be phased out shortly so you should use http://old.fastmail.fm from now on.
The old web interface is deprecated. No more development or updates are being made to it. Features will be progressively disabled where they conflict with new changes (eg database changes, IMAP server changes, etc). We highly recommend users of the old web interface switch to the new interface. The improved search and keyboard shortcuts alone are a huge productivity improvement.
Sometime soon we'll also be removing the user web interface preference, so that to login to the old web interface you will have to use http://old.fastmail.fm, using http://www.fastmail.fm will always login to the new interface. We'll be letting users of the old web interface know about that change shortly.