Firefox 8 version of TenFourFox is available. You can try the third-party product TenFourFox that supports PPC Macs. The bad news is that support for the Firefox 3.6.x series is likely to end in the first quarter of 2012. The good news is that Firefox 3.6.24 still supports OSX 10.4 and Macs with the PPC Mac architecture.
Beginning with Firefox 4, Macs must have an Intel x86 processor (which you do NOT have yours is a PPC Mac) and OSX 10.5 or higher