You decide if you want to open links in the same tab or in a new window. Click the Account icon in your top right corner, and open the Account preferences.
Locate the second toggle option, and set it to 'Open links in new window.' It will turn bright green. Now every time you click a bookmark, you'll get a new tab.
If you leave this option untoggled, it will open in the same window or tab you're currently viewing.