I am going to clarify this ahead of time as in the future you are going to be seeing the names “Minefield” and “Shredder” used quite a bit for this point on. Minefield refers to the trunk/developmental builds for Firefox 3. This is not to be confused with Gran Paradiso, which is the code name for Firefox 3. On Sunday I switched my update channel from Beta to Nightly so I could start getting and testing the latest nightly releases of Minefield instead of just using the Beta 5 release of Gran Paradiso.
Now that I have y’all thoroughly confused, let me explain Shredder. The developmental builds of Thunderbird 3 are going to be under the code name Shredder. So when Thunderbird 3.0a1 comes out soon it will (or may) be released under the name Shredder 3.0a1. David Ascher is currently seeking help for creating a Shredder Icon and artwork.
Edit: Percy gives a very good explanation of Minefield and Shredder in his article Shredder: in-development Thunderbird.