Mozilla Firefox Project (Development Information)
Firefox 1.5 Beta 1 is now available for download. This is the first Beta release of our next generation Firefox browser, to be released later this year, and it is being made available to our developer and testing community for compatibility testing and to solicit feedback.

Download here:
http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/1.5b1/

Direct link to Win32 build:
Firefox%20Setup%201.5%20Beta%201.exe