Make sure you have fully exited Firefox last by using the Quit option. In Mac OS X, applications do not automatically exit when you close the last remaining window as they do in other operating systems. If you are quite sure that you quit, it's possible that Firefox may have hung in the course of shutting itself down. In that case, you can read these instructions for 'force quitting' an applicaton: http://www.techiecorner.com/769/how-to-force-quit-program-in-mac-os-x/ Only do that as a last resort though. Finally, uninstall your existing version of Firefox before attempting to install a new version. You can do that by opening the Applications folder from the Finder, then dragging the Firefox icon to the trash can.