Apple has released firmware 7.6.7 for all of its 802.11n AirPort Base Stations. This includes the AirPort Express, AirPort Extreme, and AirPort Time Capsule.
According to Apple's release notes, it improves stability and performance for its AirPort Base Stations. It also fixes an issue which may prevent communication between clients.
Apple has also released firmware 7.7.7 for the 802.11ac AirPort Extreme, and AirPort Time Capsule. The updates can be applied by using the AirPort Utility app on iOS or OS X.