Mozilla stopped releasing the zip builds when they released new versions of Firefox due to crashes caused by faulty installations.

They do make zip builds and they are available from the same ftp sites that you get the Minefield zip builds. Ensuring that you get the one that corresponds to the release is not always straightforward. For 3.6.13 it is available from ftp://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/3.6.13-candidates/build3/unsigned/win32/en-US/

Thank you very much,I was indeed scavenging on the ftp site in search of this,but as you say stuff is not easy to find at times-I just wanted to have this build available,not as a substitute of a regular installer but just in case,so to speak.