Thunderbird 1.5.0.4 is currently undergoing internal testing, which will (hopefully) be released on 31st May 2006.
Mac-specific:
- Fixed: 327037 - [Mac-only] Newsgroup names over-abbreviated on UB Mac
No outstanding bugs.
Official
Windows, Official
Windows installer (discussion)
Official
Mac recent build unavailable, Nightly
universal recent build unavailable

cf | 28-May-06 at 6:37 am | Permalink
While you can’t get PPC binaries any more (the tinderbox Fireball was turned off), you can get a Universal nightly here:
http://ftp.mozilla.org/pub/mozilla.org/thunderbird/nightly/experimental/universal-binaries/
Hopefully these will be published to the regular location soon (bug 338353).