As of Firefox 4.0 and newer one needs both OSX 10.5+ and '''Intel''' hardware. Firefox 3.6.* from Mozilla is the last to work on PPC. http://www.mozilla.org/en-US/firefox/all-older.html (Firefox 3.6.25 was released recently). Since you have a PPC there is a third-party option for Firefox 9.0 at http://www.floodgap.com/software/tenfourfox/ however do note http://code.google.com/p/tenfourfox/wiki/PluginsNoLongerSupported if you use this build.