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 more than a year, updating with major security or stability fixes.
The schedule and details are available on the ESR official website.
We strongly encourage administrators deploying Firefox ESR to sign up to the Enterprise Working Group (EWG) mailing list.