I am at Firefox ESR 52.4. It still allows most of my add-ons to work and has the classic GUI.
I did run across a little program that enhances FF and the various renditions of it — FIREMIN. It seems to have taken control of the memory leaking Firefox has had since about version 0.7. I installed it a few weeks ago and occasionally check the FF usage through Task Manager. My FF memory usage dropped from 900meg to 15meg.
https://www.rizonesoft.com/downloads/firemin/
FYI