I have recently moved from Thunderbird to SeaMonkey - much better and up to date with the latest Mozilla code.
See my SeaMonkey page at: http://links.zero.eu.org/seamonkey/
For PPC - download the latest SeaMonkey-ppc from:
For Intel - download normal SeaMonkey from:
Old versions of Thunderbird can be found here:
The latest Thunderbird for PPC is 3.1.20.