Firefox Extended Support Release (ESR) is based on an official release of Firefox for desktop for use by organizations who need extended support for mass deployments. Unlike other release channels, ESRs are not updated with new features every six weeks. They are instead supported for around a year, updating with major security or stability fixes. The current ESR version is based on Firefox 45 issued on March 8th, 2016. The next ESR version will be number 52.
We strongly encourage Firefox ESR users to sign up to the Enterprise Working Group (EWG) mailing list.