If you use Firefox, check out this page:
This gentleman takes the Firefox source code and compiles it with optimizations for specific processors. This should mean that the Intel-specific version he supplies will run faster on the Intel-based Macs than the generic Firefox release. Ditto for the G5 and G4-specific versions.
Give it a whirl, but don’t trash your copy of Firefox until you’re sure this does all you want and doesn’t break anything.