Still trying to locate correct Firefox version to run with Mac power PC running OS 10.5.8.
Firefox 3.6.x is the last available from Mozilla for PPC Macs.
For older Macs that aren't supported in the Firefox 4+ versions, try TenFourFox for PowerPC's running Mac 10.4.11 & 10.5.8 .
Firefox 4 and above requires Mac OS X 10.5 or greater and will only run on an Intel Mac. Mac OS X 10.4 users, and 10.5 users with a PPC Mac, can run the latest 3.6 series available here.
You could also try TenFourMac, which is a fork of Firefox and is based on Mozilla 2.0 and Firefox 4 code.
Hopefully this helps!