Merci for the suggestion, but doing a refresh could wipe out a lot.
However — and I am well and truly hornswoggled — I went to about:profiles in the ESR version to get the lie of the land and then went to Firefox 67.0.4 and set the default profile to ESR's (and previous FF versions') profile.
Et voilà ! As if by magic all became right with the world — every setting, all bookmarks and other established preferences have carried over — and I'm working with it right now. That gift horse's mouth was wide open, and I don't know why that wasn't given (by Mozilla) as the preferred method for syncing stuff.
Let's just pray that it sticks.
