I think the best bet for you is use tenfourbird. http://en.sourceforge.jp/projects/tenfourbird/

It is an unofficial build of Thunderbird for PowerPC based macs hence the name. The plus side is you get to use the current version as there is a build of tenfourbird release each time Thunderbird is ungraded, building on their expertise with tenfourfox http://www.floodgap.com/software/tenfourfox/.

The upside is your get the current version of Thunderbird (31.1.1) built for the powerPC mac. I am not aware of any downsides really.

May be the same as TenFourFox in not supporting Plugins. https://code.google.com/p/tenfourfox/wiki/PluginsNoLongerSupported

Perhaps James. But if your only doing mail, Thunderbird does not support plugins in the mail body anyway. So it is only relevant is your using RSS feeds etc. If all else fails, V3.1 works for OSX10.4 but having a current Geko under the hood is more important to me than plugins. (But now I am aware of it as a downside and can pass it along)

True true.. well at least the OP knows not to expect use of Plugins if TenFourFox is used which is a good thing as they are rather vulnerable.