For what it's worth, they may not be completely gone.
When Firefox 56 came out I lost Xmarks functionality altogether, so I switched to the beta version (184.108.40.206a, which updated yesterday to 220.127.116.11a). Not only did I lose my keywords, but other small things like separators in my list of bookmarks were gone as well. But I also use FF on my work computer (which is on version 52 ESR), and there I'm using the regular old version of Xmarks and everything looks/functions normally.
So if you log in to Xmarks, you might still find them there, it's just that you can't use them with the beta version. Doesn't solve the problem but at least, hopefully, they aren't gone.