Well, there's now TenFourBird builds available from our anonymous builder in the land of the Rising Sun. I don't use Thunderbird and in fact still read my mail on a shell account, but it does start up and appear to run normally on my G5, and (s)he offers all four architecture builds, so I'm interested how you Thunderbird users find it now that Thunderbird 3.1 is shortly to end with Fx3.6.
As mentioned, I have TenFourFox 12 up and running (this post is being made in it) and a first pass at the CoreGraphics canvas backend for Azure. The Azure project is Mozilla's attempt to reformulate their graphics stack, in the same way that they did with Cairo way back in Mozilla 1.8/Firefox 1.5. Prior to that Mozilla used various system APIs and gaps in support were sometimes evident from platform to platform, so Cairo came along and offered a stable, standardized way to generate similar graphic content on the supported platforms. Mozilla started with Cairo for HTML canvases in 1.8, and in 1.9/Firefox 3.0 Cairo ended up doing everything.
Cairo can be very fast, but because it is a mid-level API, it inserts a level of complexity and therefore extra code. More to the point, it does not exactly map to the underlying operating system and the conversions needed (and the internal state it needs to maintain) can be expensive to handle. For Quartz a/k/a CoreGraphics it ends up working through two levels of abstraction and this is especially painful to us because we render in software. While Cairo made a lot of code simpler, its drawbacks are now unfortunately apparent.
So Azure tries to map closer to the underlying API, which in our case is Quartz/CoreGraphics, and allows primitives and drawing operations that map more directly (in some cases 1 to 1) to the underlying operating system. This allows us to take advantage of whatever hardware acceleration is available to Quartz, but more importantly it lets us get around a lot of unnecessary bloat. This bloat is even worse than I thought: our current CoreGraphics backend in the forthcoming 10.4Fx 12 accelerates canvas performance by almost 60% on Peacekeeper. That is a smoking hot win. Eventually Mozilla wants to do as much of the browser as possible in Azure, just as they did with Cairo, with Cairo existing as a fallback or for printing.
That's the good news, anyway. The bad news is 1) it's only for canvases right now and 2) our backend doesn't fully work; text doesn't render in canvases yet and gradients make it randomly crash, so I still have some work to do. By default it ships disabled, but you can turn it on in about:config and I will tell you how when it is available. Even in this incomplete form it will still pass Peacekeeper and run a number of demos. Don't expect night and day speeds, but you will notice a difference.
I have not yet heard when 10.0.4 will go to build, but there are a lot of fixes in it. I will make 12 available when the RC is signed off. In the meantime, post your TenFourBird comments; I'm curious how people who actually use an E-mail client will find it.