Sort of got the menu back.
Upgraded Firefox to 48.0.2.
Did this: http://geo.inge.org.uk/gme_trouble.htm#resetconfig - Be very careful, that will also loose your Bing Key, so copy it beforehand and copy any other settings you have
I now have the menu back. Of course, could be a co-incident.
However, as soon as I add gccomment I'll loose the menu again. Map Enhancements never worked with 'gc little helper', now also not with gccomment.
Any clever person out there that has a clue where the problem might lie, other than gc fiddling with their maps again?