Unfortunately, Firefox 4 supports 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 here).

You can revert back to previous version of Firefox by following these steps:

1. Open the Applications folder.
2. Drag the Firefox application to the Trash.
3. Download and install Firefox 3.6 from here.

The 3.6 downloads are just .exe files for PC - anyone know of a 3.6 download for Mac??

@jasonneo, as mentioned above, go to http://www.mozilla.com/en-US/firefox/all-older.html and select the Mac download for your preferred language.

doh - many thanks!!!!