I did/have not updated Firefox x64, on a current and always updated W10 PC, since the "new" version would have prevented the use of a couple of legacy extensions, most notably Theme Font & Size Changer 56. If I am understanding the nature of ESR correctly, security wise, my Firefox will stay up to date, is that correct? I actually did use several of the manual tweaks in an attempt for it to render it the way I currently have it set up, however, it was a little overwhelming for an old guy! Thank you!
As Brummelchen noted: It is going to receive updates for security vulnerabilities until aug'18. At least for vulnerabilities Impact key scored as critical and high. Actually sometimes they are going to push some update for moderate level security vulnerabilities or reliability vulnerability.