As of Firefox 4.0 and newer one needs at least OSX 10.5+ and Intel hardware. It used to come with 32-bit version for Intel and the PPC version. The PPC was dropped and 64-bit version was put in place.

The last Firefox version from Mozilla to work on PPC Mac is Firefox 3.6 where the recently released 3.6.25 is current. http://www.mozilla.org/en-US/firefox/all-older.html

The only other option you have besides building it yourself is trying a third-party build of Firefox 9.0 from http://www.floodgap.com/software/tenfourfox/ though do note http://code.google.com/p/tenfourfox/wiki/PluginsNoLongerSupported if you use it.

To see how it changed.

http://www.mozilla.org/en-US/firefox/3.6/system-requirements/
http://www.mozilla.org/en-US/firefox/4.0/system-requirements/

Yes, we PPC users would appreciate FX 3.6 being kept current for as long as possible! Plug-ins aside, please see the following thread for issues using TenFourFox. Basic 3.6 security maintenance would make our computers usable for much longer.

http://forums.mozillazine.org/viewtopic.php?f=23&t=2323835&sid=92c589d0b07be11743987ae53233ee15

Intel Mac OS Tiger users would also appreciate 3.6 being updated as well, since 4 and up will not work on Tiger.

Firefox 3.6 has already been supported among the longest of Firefox releases so far at almost two years now. The 3.6 release came out way back on January 21'st 2010. http://www.mozilla.org/en-US/firefox/3.6/releasenotes/

Originally it was going to be EOL shortly after the Firefox 4.0 release.

Yes, but it's not that we are in love with 3.6 for its own sake--it's that it is the only version of Firefox (or any reasonable browser) that is maintained with regular security updates for PPC. It is true that my PPC computer is six years old, but Firefox is maintained for Windows computers that are much older. So, we are grateful for 3.6.25, and hope that there will be future updates for critical security issues. We are not expecting the latest thing--just a reasonably secure out-of-date browser ;)