Apple also said there are security holes patched in the update, but the security document for this release has not been published as of this writing.
Apple’s patch notes in full:
AirPort Base Station Firmware Update 7.7.8 Information
Firmware update 7.7.8 is recommended for all AirPort Extreme and AirPort Time Capsule base stations with 802.11ac. This update addresses an issue that may prevent AirPort base stations from appearing in AirPort Utility when Back to My Mac is enabled.
You can download the update through AirPort Utility.
Warning: update failed on my Time Capsule and disappeared from my network. Had to reset power to unit and restart. Not sure I will try this again anytime soon.