All bookmarks (probably a couple thousand) disappeared Dec. 7, & Xmarks failed to restore. Nothing worked, at all. After thrashing around a bit, I found where Xmarks stores its backup synchs. The latest, dated Dec. 4, was an empty file, but Nov. 27 had them all, at least up to that date. So I opened that file in FF and saved the page as an html file to HD. Complained to Xmarks, got no answer. Likewise Mozilla. Finally today I found the import/export bookmarks function in FF (never had a reason to use it before, so I had no idea it was there), and used it to import that html file back into FF bookmarks. That actually worked fine, so all the bookmarks (up to Nov. 27) are back. Whew, to say the least...
I'm not sure that this solution is good for everyone, but it might help someone. Oh, I also turned off auto-synch in Xmarks. From now on looks like I'll just have to manually backup bookmarks to an html file. Ugh. One more chore.
So why have we been put through this crap? I'm pissed at Mozilla for just casually breaking FF add-ons in the first place, as well as not issuing warnings, or admitting on its website that they're giving us a royal pain in the butt (for something as essential as bookmarks!) while they tinker around with new code. I'm also pissed at Xmarks/LastPass for much the same reasons. They have all our email addys, so why no warnings, explanations, apologies, nothing?? Stinks.