Hello, So here is the FTP server link for the Release you are looking for you just have to go and choose you language from the page and then download the .DMG file from the server. Then your all set. Have a good one.

https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/31.4.0esr/mac/

From, Sir.Mayo

Reason for edit: My old post was not correct.

https://www.mozilla.org/en-US/firefox/organizations/all/

Firefox 31.6.0 esr is the current security/stability update as 36.4.0 esr is two updates behind.

Firefox 31.4.0 ESR:

• https://www.mozilla.org/en-US/firefox/31.4.0/releasenotes/

Version 31.4.0, first offered to ESR channel users on January 13, 2015

Version 31.6.0, first offered to ESR channel users on March 31, 2015