My Sync is set for Add-ons (which Themes is a part of), Bookmarks, Options, across (5) Windows 10 laptops and (1) Windows XP SP3 desktop. Everything syncs perfectly in all of the devices except Themes. There is one same theme for all of the devices. However, the theme in the Windows XP SP3 desktop CANNOT be changed because none of the currently available themes function in Firefox 52.9, the last version that works with Windows XP SP3. Is this preventing Themes from syncing in the Windows 10 devices? I presently change to one same theme manually in each of the Windows 10 devices, but I am looking for a workaround to make Themes sync in the Windows 10 devices.