I wonder why Mozilla doesn't mention that possibility on the ESR FAQ page? (Did I miss it?) It's certainly an excellent option for users in your position.
It's to be found elsewhere in the Mozillasphere:
Firefox support has ended for Mac OS X 10.6, 10.7 and 10.8 .
Firefox 49 requires Mac OS X 10.9 (Mavericks) or higher. If using an earlier version of Mac OS X, there are options to continue using a secure version of Firefox.
Option 1: Upgrade to an OS X version supported by Apple
In order to continue browsing safely, Mozilla recommends upgrading to an OS X version that Apple supports. While Firefox will still continue to work on unsupported versions, using Firefox on a supported version will provide the best and safest user experience.
Option 2: Download Firefox Extended Support Release (ESR)
Firefox Extended Support Release (ESR) version 45 will continue to receive updates on OS X 10.6, 10.7 and 10.8 until 2017. Version 45 is the last ESR version to receive updates. In 2017, ESR 45 will no longer be supported. See the Firefox ESR page to download Firefox ESR.
Following are the release dates of the last versions of ESR:
2016-12-13 Firefox ESR 45.6
2017-01-24 Firefox ESR 45.7
2017-03-07 Firefox ESR 45.8
2017-04-18 Firefox ESR 45.9
