I finally got the blasted thing working (I guess) this morning. I've looked in logs and prefs. I disabled all extensions in FF56.0 and only enabled Xmarks. I even opened Tools/Options/Privacy&Security and clicked "Saved Logins" to see if there was anything in there (there wasn't). Cleaned and re-started. There is a large (32 character) alpha-numeric cookie that I "kept" and now, after much travail, seems to sync my bookmarks. So I guess it was the weird "cookie" and not a FF profile issue. I'm not really sure at this point. Will cross fingers.
Lastpass / Xmarks used to be a good company, but now seems flaky and unconcerned about communicating with it's users. I know that there will be a lot of confusion in the next several versions of FF and I can't wait until things stabilize in Jan/Feb.
Thank you all for your help! I consider this issue solved and really a Xmarks code problem.