Dropping Mac OS X 10.4 support in Gecko 1.9.3

In September of 2009 we stopped supporting Mac OS X 10.4 ("Tiger") on
mozilla-central but we left much of the code required to support that
platform in the tree in case we wanted to reverse that decision. We
have come to a point where we need to make a final decision and either
restore 10.4 support or remove this (large) amount of 10.4 specific
code.

Here are usage numbers from 2010-01-25:

===========
Firefox 3.5
===========
10.6 (Darwin 10.x): 1,497,221 (26%)
10.5 (Darwin 9.x): 2,855,842 (50%)
10.4 (Darwin 8.x): 1,379,770 (24%)
All versions of Mac OS X: 5,732,833

===========
Firefox 3.6
===========
10.6 (Darwin 10.x): 186,825 (59%)
10.5 (Darwin 9.x): 91,478 (29%)
10.4 (Darwin 8.x): 35,960 (12%)
All versions of Mac OS X: 314,263

Mac OS X 10.4 was released in April of 2005 and a lot has changed
since then. We would like to take advantage of more modern
technologies on Mac OS X and 10.4 support has been a hindrance. Where
we can work around supporting 10.4, doing so consumes valuable time
and effort. Neither Chrome nor Safari has to deal with this.

The approximately 25% of our Mac OS X users still on 10.4 would
continue to be supported by Firefox 3.6 until that product reaches end
of service, which won't be until several months after the next major
version of Firefox is delivered (built on Gecko 1.9.3) later this
year. Past data shows that we do not lose appreciable market share
when we stop supporting a Mac OS X version. We are often one of the
last vendors to continue supporting older Mac OS X releases, and I
suspect that by the time this becomes an issue Apple may themselves
have stopped issuing security updates for Mac OS X 10.4.

Adding 10.4 support back to mozilla-central would mean switching back
to ATSUI from Core Text, switching back to gcc-4.0 from gcc-4.2, and
doing a bit of porting work for code that has been added to the tree
since we dropped support for 10.4. Other areas where 10.4 support
consumes our time, makes our code more complex or error-prone, and/or
limits our capabilities include complex text input (IME), out-of-
process plugins, printing, native menus, and Core Animation.
Furthermore, Apple's upcoming JavaPlugin2 will not support Mac OS X
10.4.

We are planning to make the decision to remove 10.4 support final and
remove the code from the tree. If you have any strong objections
please let us know now.