Firefox 4 requires at least OS X 10.5 and an Intel Mac. There is a third party version of Firefox 4 that runs on OS X 10.4/10.5 and PPC Macs, for details see http://www.floodgap.com/software/tenfourfox If you prefer, you can get the latest version of Firefox 3.6 from http://www.mozilla.com/en-US/firefox/all-older.html If you have OS X 10.3.9 or earlier, the last version that works is Firefox 2.0.0.20 available from https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/2.0.0.20/mac/en-US/