PPC Mac's aren't compatible with the Firefox 4+ versions, they require Intel architecture.

For older Macs that aren't supported in Firefox 4+ versions, try TenFourFox for PowerPC's running Mac 10.4.11 & 10.5.8 .
http://www.floodgap.com/software/tenfourfox/

http://tenfourfox.blogspot.com/

http://www.mozilla.org/en-US/firefox/4.0/system-requirements/

to
http://www.mozilla.org/en-US/firefox/8.0/system-requirements/

Mac Operating Systems Mac OS X 10.5 Mac OS X 10.6

Recommended Hardware

Macintosh computer with an Intel x86 processor
   512 MB of RAM
   200 MB hard drive space

Firefox 3.6.* and earlier used to come with Intel and PPC versions in download, but now as of Firefox 4.0 and newer (including 8.0) the PPC version was replaced with 64-bit version.