As far as I can tell, the Australis work dates back roughly to 2012 (that's when their wiki meeting notes start, and when the Australis tracking bug in Bugzilla was opened).
You may be mixing Australis up with the theme work done for Firefox 4.0 (which moved tabs to the top by default and added the orange Firefox menu button). The early work on that would've been contemporaneous with the earliest public versions of Chrome (2/3/4)-- the design that would become Firefox 4 started being shown off in public around the fall of 2009.
You may be mixing Australis up with the theme work done for Firefox 4.0 (which moved tabs to the top by default and added the orange Firefox menu button). The early work on that would've been contemporaneous with the earliest public versions of Chrome (2/3/4)-- the design that would become Firefox 4 started being shown off in public around the fall of 2009.