Unfortunately, Firefox 4 and 5 support only Mac OS X 10.5 and above on Intel hardware. The system that you are currently using is no longer supported by Firefox (details can be found [http://www.mozilla.com/en-US/firefox/system-requirements.html here]). <BR><BR>Since Firefox 3.6 is still being maintained and supported by Mozilla, you can upgrade to it by following these steps: # Open the '''Applications''' folder. # Drag the Firefox application to the '''Trash'''. # Download and install Firefox 3.6 from [http://www.mozilla.com/en-US/firefox/all-older.html here]. Note that, in future, if Mozilla stops maintaining or supporting Firefox 3.6, you have the option of using another Firefox-like browser on your current system ([http://www.floodgap.com/software/tenfourfox/ TenFourFox]). Keep in mind that TenFourFox is not developed or endorsed by Mozilla in any way.