PC-Sicherheit.net

Release dates:
Firefox 55 / ESR 52.3 release date: 2017-08-08.
Firefox 56 / ESR 52.4 release date: 2017-09-26.
Firefox 57 / ESR 52.5 release date: 2017-11-14.
Firefox 58 / ESR 52.6 release date: 2018-01-16.
Firefox 59 / ESR 52.7 / ESR 59.0 release date: 2018-03-06.
Firefox 60 / ESR 52.8 / ESR 59.1 release date: 2018-05-01.
Firefox 61 / ESR 59.2 release date: 2018-06-26 ->end of life for legacy add-ons

Note:
If you are on Firefox ESR 52.6, you should get an offer to upgrade to Firefox ESR 59.0 on 2018-03-06. If you decline that offer, Firefox will just update to Firefox ESR 52.7.
This will be repeated for one more cycle (Firefox ESR 52.8/59.1). From 2018-06-26 on Firefox ESR 59.2 will be the only up-to-date and "secure" ESR version of Firefox and ESR 52.x will be deprecated.