Actually, checking now -- having left one tab, with only this HN thread left open overnight (and computer closed) FF's memory usage has crawled back up to 2.88GB.
In "Explicit Allocations", 488MB of that are "heap-overhead". 455MB go to js-non-window" , of which 254MB are "zones" and 183MB "runtime". "window-objects" take 279MB, (the js-compartment for about:blank takes 151MB). Another 259MB for "heap-unclassified", and 155MB for "gfx".
In "Other measurements", there are 745MB of "decommitted", of which almost all is "js-non-window". "js-main-runtime" is 666MB, split evenly between "compartemnts" and "runtime" or "zones".
Nothing else is of much significance.
No extensions or plugins seems are explicitly noted in the memory breakdown. In my experience GC, CC, and Minimize Memory Usage make no difference. I do make use of Noscript and Privacy Badger, and I suppose I should try disabling them -- but my gut says disabling plugins that block scripts won't reduce JS memory usage.
Pretty much every time I've checked it really looks like oodles of JS-related memory leakage.
Anyway, not expecting a diagnosis here -- this isn't the appropriate forum for that. Just venting a bit. Thank you for your suggestions!