That download link seems to point to Version 2.
Try this one for 1.5.0.x:
http://www.mozilla.com/en-US/firefox/all-older.html
I didn't realize that they would continue with updates on the older version.
Quote for site:
Quote:
Firefox 1.5.0.x will be maintained with security and stability updates until April 24, 2007. All users are strongly encouraged to upgrade to
Firefox 2.
I also noticed that they have just released a new version of Mozilla Thunderbird 1.5.0.8
Release Date: November 7, 2006
http://www.mozilla.com/en-US/thunderbird/