Doubt it, but here's a good test idea. Download another version of Firefox(Beta,Dev,or Nightly), make sure to install in a different directory than your current install. Then sign into your Mozilla account in the new Firefox, uncheck the bookmarks option in the sync settings. Then create a few test bookmarks in the new Firefox and they should not sync back to the current Firefox. You'll then have to wait and see if the sync option from your current Firefox puts the check back into the sync bookmarks option in the new Firefox. https://www.firefox.com/en-US/channel/desktop/?redirect_source=mozilla-org