If Safari is your default web browser, webpages open in Safari when you click links in other applications, such as Mail. You can specify whether these pages open in a new window or a new tab in the current window.
Choose Safari > Preferences and click General.
Next to “Open links from applications,” choose the option you want.
This setting applies to links found in other applications, such as email messages and text documents. To open a link on a webpage in a new tab, Control-click the link and choose “Open Link in New Tab.”