Mozilla will offer an Extended Support Release (ESR) based on an official release of Firefox for desktop for use by organizations including schools, universities, businesses and others who need extended support for mass deployments. Firefox ESR is intended for groups who deploy and maintain the desktop environment in large organizations such as universities and other schools, county or city governments and businesses.
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
Mozilla Firefox Extended Support Release (ESR) is, essentially, a slower-moving, more stable version of the regular browser. While standard Firefox sees a major release every 6 weeks or so, Firefox ESR will only move a whole release number every 54 weeks, meaning that you're much less likely to find some new tweak has broken your favourite extensions.
Of course this doesn't mean the browser will see no movement at all for an entire year. Every time standard Firefox is updated, Firefox ESR will incorporate all the latest security updates in its own minor upgrade, so the program will be every bit as safe to use: you just won't have all the latest features.