Where is the setting, and is it global to the browser or configurable by site?
It is a Global setting. You can configure it to open links to a different web site in different tabs or ALL links in a new tab.
Go to the Firebird Extensions web site and download TABBROWSER EXTENSIONS. When installed open the options window in Firebird....go to Extensions......highlight TABBROWSER EXTENSIONS....click on settings and there will be tons of options on how to configure firebird/tabs.