As FF 57 inevitable release loomed closer, Xmarks began to suffer quite an amount of problems:
1. It stopped saving tags; to me, it's even bigger disaster than losing bookmarks (please note that FF, by default, keeps several daily backups). Not fixed. I have to restore approx. 5,000 tags.
2. It began constantly crashing, especially for larger bookmarks set (mine has approx. 3,000 bookrmarks ATM). Partially fixed.
3. It began duplicating bookmarks added on another devices, when syncing. Not fixed.
4. Now, starting with the last 126.96.36.199 version, it began to lose bookmarks. Not fixed.
A huge disappointment, it is now. I do not know whether they do any testing; looks like the company began hastily re-writing the plugin, when FF 57 release began really near, and quite forgot about any kind of testing.
Well, I have to return to old semi-manual bookmarks merge/copy approach.
I hope their LastPass won't get the same disappointment.