Well, Firefox forced my hand by upgrading even though I had “Never check for updates” checked. So I now have Firefox ESR installed. I posted recently about why I’m not ready to upgrade.
The downgrade seems to have lost all of my history and like a month of tabs. If I open the same profile in current Firefox, I can see the newer tabs, but not the tab groups other than the first one. So I may have to switch between the two for a while. Either that or find a copy of 56, then do a manual back-up of my tab groups to re-import in ESR.