You can get an unofficial version of Firefox 4 that runs on Mac OS X 10.4 from http://www.floodgap.com/software/tenfourfox/

Or you can re-download Firefox 3.6 from http://www.mozilla.com/en-US/firefox/all-older.html

Thanks, mbrubeck. I finally was able to reload Firefox 3.6 after a couple of tries and everything's fine. I was skeptical, but this problem solving system worked perfectly well for me this time.

Perfect solution! Thank you for the link to 3.6. Exactly what info I needed. I'm running Mac OS X 10.5.8. Firefox 4.0 is not supported. Back at 3.6. Thank you!

What is going on? I can't believe that it's virtually impossible to find a place to download the old version without going through all kinds of hoops and hoping that someone was brave enough to face the confusion of getting to the right place in order to find a replacement version. Why is system compatibility check not available before downloading? 4.0 downloaded and installed without ever stopping to check compatibility, which would have saved several hours of trying to figure out how to get the old version back (and it's no easier when you have a brain injury believe me!)

And I'll still be forced to use this sucky new version in a "short time?" Have you people totally gone off the rails?