Not a solution for everyone or even most; but my workaround is go back and use Firefox 55 and Xmarks 4.4.1
1. download old version of firefox, 55.0.3 https://ftp.mozilla.org/pub/firefox/releases/55.0.3/
(for Windows, look in either win32 or win64 directory)
2. Remove add-on xmarks. exit, restart firefox
3. Uninstall firefox. restart PC.
4. install firefox 55.0.3 (It should re-use same profile, maintain your bookmarks, add-ons, etc )
5. install older Xmarks add-on, v 4.4.1 https://addons.mozilla.org/en-us/firefo ... /versions/
6. Under firefox, Tools, Options, advanced (left margin panel), Update (top bar tab), choose "Never check for updates (not recommended: security risk)"
7. Wait for a few days, weeks?, month(s)? for these issues to get straightened out and for xmarks Webextensions version and latest firefox to become reliable again. Then update firefox and xmarks. I realize that this places me at security risk in the meantime.
I use Xmarks to sync my firefox history, and don't use it to sync bookmarks. When I tried using newest Xmarks 18.104.22.168, I do not know if there is an option to turn off Bookmark sync and choose only History sync, so I am reluctant to use the newest version.