Hi If you type '''about:preferances#sync''' in the address bar, you can then select to disconnect your Firefox account. This will still leave your bookmarks on the "old" computer, that you will need to delete manually. Sync is not cloud storage or a backup mechanism, so a copy of the data will still be on your old machine for you to delete in the normal way. When getting rid of an old computer you are likely to have other personal information that should be be securely erased (or ideally destroy the old hard disk). Dedicated Windows support forums should be able to help you with this.