Firefox 4 versions and later do not run on a PowerPC Mac.<br /> Firefox 4 and later require at least OS X 10.5 and an Intel Mac. * http://www.mozilla.com/firefox/6.0/system-requirements/ For an unofficial Firefox 7 compatible version (TenFourFox) that runs on PowerPC Macs with OS X 10.4.11 or OS X 10.5.8 you can look at: * http://www.floodgap.com/software/tenfourfox/ * http://www.macupdate.com/app/mac/37761/tenfourfox Firefox 3.6.x (Mac OS X 10.4 and later) can be found here: * http://www.mozilla.com/en-US/firefox/all-older.html