You are using Mac OS version 10.3.9. Unfortunately, Firefox 3.6.13 does not suppot it. You need to have atleast Mac OS 10.4 or above.
Firefox 2.0.0.20 is the last version that runs on OS X 10.3.9.
You can try other browsers like Opera 9.

I have not received an answer to my question. I have downloaded Firefox 3.6 to my Mac OS 10.5

It sits on the desktop and does not replace The original-It says I already have one version active and can not open the 2nd.

How do I close the original and replace it with the newer version and retain all my bookmarks and files associated with the older version.

@slonde:

• Download a new copy of the Firefox program: http://www.mozilla.com/firefox/all.html
• Trash the current Firefox application to do a clean (re)install.
• Install the new version that you have downloaded.

Your profile data is stored elsewhere in the Firefox Profile Folder, so you won't lose your bookmarks and other personal data.