I noticed the same problem, just on a smaller scale with the inability to delete or empty the bookmark trash.  I am over my trash quota because I can't empty my bookmarks.  How do you fix this problem?  I was able to successfully delete the Speed Dial links that were in the trash.  The delete process was inefficient since I could only delete 1 Speed Dial at a time.  I would recommend implementing a "Delete All" button to facilitate this function.

Check your Firefox Preferences in the "Firefox Account" then under Sync Options - uncheck your "Bookmarks" and save.  When I used Xmarks, their documentation was clear that you do not want two sync processes of bookmarks going on at the same time (i.e. Firefox native mode and Xmarks) because this could cause duplicate bookmarks, folders, etc.  When I switched to EverSync, I left the same settings I used for XMarks and have not had any problems.  Try it to see if it helps.

FYI, have you found any documentation on EverSync?  I don't see any recommendations from them on how to set up the extension.