Ok, you've got jQuery as one of your tags, so try this link: Compare 2 arrays which returns difference.
It returns the differences between two arrays. You'll have to perform this twice for what you're doing, once to figure out what is in current
that is not in previous
and vice versa. I don't know what properties are contained in your bookmarks so this simple example might not exactly suit your needs, but it might point you in the right direction.
Good luck and welcome to JavaScript!