See Install an older version of Firefox

It would normally be unwise to use old unsupported and insecure versions of Firefox on the internet. Look at all he security fixes that are now public knowledge and therefore potential exploits

• https://support.mozilla.org/questions/new
• https://www.mozilla.org/security/announce/

The document I linked to first of all contains a link to the archived versions.

I can not remember the differences between the versions now, but may be able to look them up. May I ask what your interest is in such an old version?

Firefox 3.6.28:

• https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/latest-3.6/

• http://www.mozilla.org/en-US/firefox/3.6.28/releasenotes/
• http://www.mozilla.org/en-US/firefox/3.6.28/system-requirements/

Firefox 3.6 is released 3 years before.
Mozilla stopped supporting firefox 3.6

Try with Firefox latest version

• www.mozilla.org

The Firefox 3.6.* updates were basically security and allowed stability updates for 3.6. It was as of Firefox 3.6.4 that the Plugin container started and was the only 3.6.* to get features.

http://www.mozilla.org/security/known-vulnerabilities/firefox36.html

Firefox 3.6.28 was released on March 13th, 2012 and is behind security fixes since then.