The latest versions of Firefox require at least Mac OS X 10.5 and an Intel processor. You can revert to Firefox 3.6.xx for Mac 10.4 by downloading the installer from http://www.mozilla.org/en-US/firefox/all-older.html More information in the [[Firefox will not start]] article, under [https://support.mozilla.com/en-US/kb/Firefox%20will%20not%20start#w_firefox-will-not-start-4on-os-x-10-4-or-earlier-or-with-a-powerpc-processorsf5on-windows-98me-or-earliersf Firefox will not start on OS X 10.4 or earlier or with a PowerPC processor]