Released on July 15 to fix sleep/wake issues and a bug that caused fans to run at full speed after waking from sleep, the EFI update has caused some MacBook Air machines to shut down completely for several hours and on other machines, the update is still listed in the App Store after being installed.
Multiple users have said their machines turned off and did not reboot for several hours after attempting to install the update, while others have been unable to install the update at all. Some users have also reported completely bricked machines even after several hours.
One user attempted to install the update multiple times with no luck, while another user took his machine to the Apple Store where employees were also unable to get the update to install.
I had the same issue. The machine simply shuts off and doesn't restart so the firmware update is never applied. The power button stopped working and the machine would not turn on at all. So I took it the the Apple Store and they cracked it open to do a physical SMC reset.
It booted up fine after that. We tried the install again and no dice. Same thing happened. This time the guy at the Genius Bar did some kind of "soft" SMC reset. He pressed SHIFT+FN+CONTROL+POWER at the same time (the first 3 keys are in the lower left corner) ... held it for a few seconds and let them go. When he pressed the Power button after that the machine booted up fine. Still didn't apply the firmware update though. The guy at the Genius Bar said he would report this issue to Engineering to alert them to the problem with the update.
It is not clear if the issue is widespread or limited to just a 2011 MacBook Air users, but as none of the affected users in the thread have discovered a solution, those who have not yet attempted to install the 2.9 update might wait to wait for an official fix from Apple.