Firefox 3.6.x is the last available from Mozilla for PPC Macs.
http://www.mozilla.com/en-US/firefox/all-older.html

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 .
http://www.floodgap.com/software/tenfourfox/
http://tenfourfox.blogspot.com/
https://code.google.com/p/tenfourfox/wiki/PluginsNoLongerSupported
http://code.google.com/p/tenfourfox/wiki/AAATheFAQ

Hi BobMcD,

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!