Apple today re-released its EFI Firmware 2.9 update for 2011 MacBook Air models after the original EFI update appeared to be causing trouble for many MacBook Air users.
The update, originally released on July 15 to address sleep/wake issues, caused some MacBook Air machines to shut down completely for several hours. On other machines, the update simply refused to install. Today's re-released software, labeled 2.9.1, presumably fixes these issues, allowing the update to be properly installed.
Article Link: Apple Launches New EFI Update 2.9.1 For 2011 MacBook Air to Solve Installation Problems