Right… Spoke/wrote/apologized/rejoiced too soon. Firefox settings don’t work, although I can migrate the .thunderbird settings.
While logged out of Firefox Sync deleted the .mozilla directory and opened up a new Firefox, opened a few windows and changed a few settings. After that copied the .mozilla directory to a new machine. No joy. Aaaargh.
Maybe this copy of the .mozilla folder has been written over too somehow while I’ve fiddled with Firefox Sync.
Oh well, if I can’t get it to work, I’ll just sign in to Firefox Sync. It’s actually pretty cool.