(Interestingly, that list of security fixes above includes several vulnerabilities in Apple Type Services, of which at least a couple likely are in 10.4 and 10.5 in some manner. However, Mozilla includes a type sanitizer to guard against exactly this sort of malicious font data attack, so TenFourFox, at least, cannot be exploited in this manner even if a PowerPC-specific attack were designed.)
Since 10.6 can't run on any Power Mac, I'm not going to bemoan its loss specifically except to say that my token Intel Core2Duo mini circa 2007 runs 10.6.8 and will continue to, running Linux and Windows 7 in virtual machines; I despise Lion and (to a lesser extent) Mountain Lion and Mavericks for a variety of reasons, and Snow Leopard is the last hurrah for any PowerPC support in OS X thanks to Apple refusing to pay IBM/Transitive any more per-user seats for Rosetta. The real relevance to us is whether Mozilla will continue to support 10.6, because once 10.6 is dropped all supported Macs will be 64-bit and have video acceleration, two things that do not apply to our userbase, to say nothing of the widget changes which will no doubt follow and will almost certainly impact compatibility.
Mozilla's decision to withdraw support has historically been a function of the remaining userbase and what competitors have done. For example, when they dropped 10.4 for Firefox 4, the Tiger install base had already been declining slowly for some time and Google had never supported Chrome on 10.4, even on Intel. Interestingly, Apple is discontinuing Snow Leopard support at a time when 19% of the Mac world still uses it (thanks for the shout-out, Ars Technica!), and in fact this number has declined very little from November 2012 when 10.6 commanded around 21%. We can speculate about why it continues to hang on in the comments, but the most frequently cited are disgust over Lion's changes, hardware incompatibility and need for PowerPC support. Because Safari no longer gets updates, as a result both Firefox and Chrome (which also still supports 10.6) are likely to accumulate large numbers of Snow Leopard holdouts: almost exactly a year ago, 43% of Firefox's users were on 10.6, and assuming the decline is proportional, even in the worst case a whopping 39% of Firefox's users are likely to still be rocking the Snowy today. For that matter, it is also entirely possible that proportion may actually increase in the near term from displaced 10.6 users seeking a safe haven. Until Google decides to kick out 10.6 users, it is very unlikely Mozilla will do so unilaterally either, or at least not until somewhere in the 38ESR timeframe. And ultimately that's good news for us.
I am slowly working through the 29 port. Mozilla not only landed Australis but also made further changes to font support, ripped out the old Color Manager code we use for colour management (so I had to restore that) and upgraded libvpx which upset our custom AltiVec acceleration and I'm still labouriously trying to piece that back together. Meanwhile, Operation Garbage Collect Better is now at the following settings: