Your system should indeed be a dual CPU. TenFourFox has no
control over that, and it is entirely possible the performance
problems you're experiencing may actually be impending hardware
failure. On my quad G5, System Profiler reports four CPUs (two
cores each on two physical processors).
For the record, I routinely use a 1GHz iMac G4 with TenFourFox.
It's not super speedy, but it's certainly functional.