http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/1.5/ Download Link(en-US): http://ftp.mozilla.org/pub/mozilla.org/firefox/releases/1.5/win32/en-US/Firefox Setup 1.5.exe
Just as a heads-up, there seems to be a bug with installing extensions with this new version. For the work-around, you'd need to look here: CLICKY!