Gecko 1.9.3 (mozilla-central) will only build and run on Mac OS X 10.5 or higher as of last Tuesday, September 15th. We will not be removing 10.4 code from the tree until we have verified benefits and made the decision final.
Motivations behind this move include performance, features, and development efficiency. By making Mac OS X 10.5 our minimum supported version we can upgrade our compiler to gcc-4.2 (from gcc-4.0) and use the Mac OS X 10.5 SDK. We can also use newer and more efficient technologies like Core Text, and add features like Cocoa-based printing and better international input support via Text Input Services. Since our code complexity will go down we can fix bugs and deliver features to the majority of our users faster.
We have not turned on the Core Text back-end or Cocoa-based printing yet, but we will be doing that soon.