Our "new" web interface was rolled out almost 2 years ago now. In that time, the vast majority of users have switched to using the new interface. It has significant improvements over the old interface, such as keyboard shortcuts, better searching capabilities, more customisation options, and all new features are developed exclusively for the new interface.
At the time of the rollout, we allowed people to choose which interface they preferred to use via a preference option. However maintaining the link that allows switching between the old and the new interface has been tricky and messy.
Because of that, we're now completely separating the interfaces. Users that wish to use the old interface must now explicitly login at http://old.fastmail.fm. The preference set on the Options -> Account Preferences screen will no longer work.
Business/family users will also have to use http://old.fastmail.fm and use their full login name.
Please note the old interface is deprecated. At the time we launched the new interface we said "We plan to keep [the old interface] for at least 6-12 months." We've supported the interface significantly beyond that time, and will continue to support it for some more months, but it will be shut down later this year.