Thanks for bringing up TenFourBird. I was interested in that once I read this post, but unfortunately, I can't seen to find a working download for it anymore. (Maybe I'm not looking hard enough?)
I was surprised, but indeed TenFourBird 38.9.0 seems to be nowhere to be found anymore out there on the wild, wild web. So, I have taken the step of adding it to my www.retro-computing.com software archives. In this case, I added it to the "Featured Software" archive, from which you can download it:
This page also features a small assortment of other titles. Please be aware that not all of them are Mac-related! The first entry on the list is QDTOOLS.ZIP, an incredible tour de force by long time DOS software author "PhG". QDTools is comprised of over 240 individual DOS (and a small number of WinNT and Win9x) utilities. DOS software is not likely to garner a lot of interest here on MacRumors, and so I thought I would save you the headache of downloading QDTools only to find that it is not relevant (unless you happen to also enjoy vintage DOS PCs in addition to Macs).
So, to download your copy of TenFourBird 38.9.0, go to www.retro-computing.com and click on the "Featured Software" icon near the bottom of the page. You will be presented with another page, whose screenshot appears above. Each software title is a link to its download page, so just click the TenFourBird-39.9.0.zip link and then click the large download icon that appears on the presented page. Voila! You now have TenFourBird-38.9.0.
I have included both the G4 and G5 variants of TenFourBird 38.9.0 and some useful additions to the prefs.js file and the userchrome.css file. The prefs.js additions allow you to control font sizes used throughout the TenFourBird GUI. The userchrome.css additions change the boring flat black icons on the TenFourBird button bar to nicer more colorful icons. You dont need to use either of these additions (prefs.js, userchrome.css); they are just included if you should want to tinker around with the GUI presentation.
One last thought. If you are using Thunderbird (or its improved Mac variant, BetterBird) on your current Mac, you will find that moving any email that you have archived in local folders on your Mac from the current Thunderbird install to your G5 TenFourBird 38.9.0 install is not at all straightforward. The seemingly obvious "export the email from Thunderbird, import it into TenFourBird" doesn't work. TenFourBird is too old to understand the export format that the current Thunderbird uses. I have a "recipe" for getting around this, and will be happy to share if you find yourself in need of it. Just let me know!