Hello, You can get it here: [https://ftp.mozilla.org/pub/firefox/releases/31.8.0esr/] First click on the directory that matches your OS / Architechture, and then choose your language.

The 31.8.0 esr security update for the old 31.0 ESR branch came out July 2, 2015. It was made EOL when Firefox 38.1.1 ESR was Released and 38 ESR is EOL due to 45.0 ESR being out with current at 453.0esr which came out when 48.0 did. Any particular reason why you need a old vulnerable version of Firefox? If you must use a version of Firefox 31 esr for a website for example then you may want to use the portable version and use Firefox 48.0.2 or 45.3.0esr for everything else to be up to date security wise. The portableapps version is self contained (even run completely off a usb flash drive if you want) and will not touch the Profiles you are using with your desktop Firefox on Windows. Portable apps did not make a 32.8.0esr build but they did make a 31.7.0esr build. The en-US locale is last one on list. http://sourceforge.net/projects/portableapps/files/Mozilla%20Firefox%2C%20Portable%20Ed./Mozilla%20Firefox%20ESR%2C%20Portable%20Edition%2031.7.0/