Older releases have known security vulnerablities, which are disclosed at 

  http://www.mozilla.org/projects/security/known-vulnerabilities.html#Firefox

Mozilla strongly recommends you do not use them, as you are at risk of your computer 
being compromised. 

For historical purposes, older builds are available at

  ftp://archive.mozilla.org/pub/mozilla.org/firefox/releases/

  http://archive.mozilla.org/pub/mozilla.org/firebird/releases/
  http://archive.mozilla.org/pub/mozilla.org/phoenix/releases/