I've written my own now, which just approximates the Chrome behavior. (I looked at Conex after you linked it, and it didn't quite do what I felt was right.)
https://github.com/kemayo/firefox-sticky-containers
I'm currently seeing whether #webextensions on irc.mozilla.org feels like ripping it to shreds, but I'll put it up on AMO after that's done.
Okay! Here it is: https://addons.mozilla.org/en-US/firefox/addon/sticky-contai...
https://addons.mozilla.org/en-US/firefox/addon/conex/