Beta 8 is now available! Go get it! Like before, we release slightly ahead of Mozilla, so it will still appear as "b8pre" in the version string.
Specific to 10.4Fx, this version now offers four separate builds: G3, G4/7400 and G4/7450, and G5. The G5 build is the most unusual since the source is very unstable built for 64-bit PowerPC, so simply -mcpu=G5 or -fast will not work. Instead, this build actually builds for G4/7450, and simply turns on the extra G5 instructions, while including -mtune=G5 which does work (effectively making it 32-bit PowerPC with the PPC 970 instructions bolted on, tuned for the G5's instruction dispatch). Together, these tweaks squeeze out about 15% better performance on a G5 than the old unified 7450 version and is steady as a rock. There have also been performance improvements to the G4/7450 version itself, which is now offered specifically for G4e systems.
10.4Fx also adds support for font tables, enabling many of Firefox 4's marquee font capabilities; enables update checking (downloads are still manual, but you now get background notifications when new versions are ready); makes some UI adjustments and fixes all the known 10.4Fx-specific crash bugs. Note that there are still some Mozilla-generic crash bugs out there and this version is still susceptible to them. That said, I've been using beta 8 as my sole browser on my quad G5 and my iBook G4/1.33 for the last couple weeks and it hasn't crashed a single time on either computer.
Have at it!