This is *not* something I'm working on, and I don't even know who this person is, but someone in Japan took the TenFourFox ESR changesets, modified and applied them to Thunderbird, and created TenFourBird. It comes in all four architecture builds and will run on 10.4 and 10.5. It even comes with an Enigmail build which you can immediately install for all your crypto needs.
If you hate the TenFourFox icon, you will find this icon absolutely hideous. That said, I fired it up on my G5 and it seems to work. I only read E-mail in a shell account, so I can't test it much more, but for those of you worried about an E-mail client now that Thunderbird 3.1 is due to end with Firefox 3.6 now have a new option.
If someone enterprising wanted to port SeaMonkey into a "TenFourApe" or whatnot, this would be a great place to start because the TenFourBird changesets already have all the needed changes for mailnews. You'd just have to do the rest of the SeaMonkey custom code which is not very much left over.
I am personally very pleased to start seeing downstream projects emerge from TenFourFox, and I hope there are others.
http://en.sourceforge.jp/projects/tenfo ... 55239/note