http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/1.5/
^there's a direct ftp, its in release/dir
you can also get it directly from http://www.mozilla.com/
Enjoy!
The release note that you see from the menu option under help:
http://www.mozilla.com/firefox/releases/1.5.html
Comprehensive 1.5 changelog:
http://www.squarefree.com/burningedge/releases/1.5-comprehensive.html
I got the ftp dl link and heard about the release of 1.5 here:
http://www.cheapassgamer.com/forums/showthread.php?t=76032
I looked up the rest of the info