Download Firefox from non-Mozilla sites is very dangerous. They could be bundling malicious software in it or tweaked it in a way that ruins performance. Then people blame Mozilla, because they thought it was the same Firefox offered by Mozilla. :( Among the changes in the latest version is also fixes to newly found security vulnerabilities, so downgrading is never recommended. If you can provide details about what problems you're having with the latest version, we can help you fix it. and that would be the better way to go. If you're still adamant about downgrading, You can get them from http://releases.mozilla.org/pub/mozilla.org/firefox/releases/