I have a bookmark backup json file (thankfully). Basically I can start with a fresh install of firefox (PC or linux), link it up to my sync account and ensure that everything is cleared out in bookmarks. Restore the bookmarks using the json file and they all appear fine. Perform a fresh install of firefox on a new computer (Linux or PC), sync the account and the bookmarks become mucked up on both machines (some bookmarks are removed others are duplicated). Clearing down both firefoxes and restoring the bookmarks from the json backup only temporarily fixes the issue (until the next time it attempts to sync at which point it mucks up the bookmarks again). Have tried clearing the profile and places but the error always occurs when syncing. I think this might in part be due to the bookmarks menu being included on the bookmarks toolbar line but removing this does not always seem to solve the issue.