Rosetta, at least from the little amount of documentation Apple published, appeared to have issues with things that generate PowerPC code and then execute it. However, TraceMonkey worked fine with Rosetta. JaegerMonkey just appears to be too much for it, though. I'll probably document this in the FAQ.
In answer to your other question, it will not compile on 10.4, and it probably won't compile on 10.5 either, because as of Fx18 gcc 4.4 is the minimum requirement (or Clang). The TenFourFox changes add a lot of hacks to get it to run on gcc 4.0.1. However, the amount of hacking I had to do for 17 and some of the miscompilations gcc made scared me greatly, and we're moving to a 10.4-built gcc 4.6.3 for Fx19. The stable branch of TenFourFox 17 will be the last version that will build with Apple-shipped gcc.
In answer to your other question, it will not compile on 10.4, and it probably won't compile on 10.5 either, because as of Fx18 gcc 4.4 is the minimum requirement (or Clang). The TenFourFox changes add a lot of hacks to get it to run on gcc 4.0.1. However, the amount of hacking I had to do for 17 and some of the miscompilations gcc made scared me greatly, and we're moving to a 10.4-built gcc 4.6.3 for Fx19. The stable branch of TenFourFox 17 will be the last version that will build with Apple-shipped gcc.