I am not an avid user of xmarks and normally I would suggest to go to the add ons support site to find more information. However I checked there and there is no response yet. In the past the old sync did not work if this add on was installed. However I have not tired this with the new sync. Please try to disable it if you have set it up to see if there is an issue there. The other issue I can see is the access to the bookmarks file in the Firefox profile, but this is beyond my troubleshooting skills with plugins. You can check to see if places.sqlte has system permissions perhaps?