Mozilla Firefox ESR (Extended Support Release) is an older build of Firefox updated roughly every 42 weeks for major changes and every 4 weeks for minor changes.
In other words, Mozilla Firefox ESR is designed to be three versions behind the current official release but with the latest security updates. It's for anyone who needs to use a previous build of Firefox safely. While you can download older builds manually going back to the first Firefox, they won't have the latest security updates.
Aside from the security updates included, you'll most likely not even notice that Mozilla Firefox ESR is a few builds behind.