In Firefox there is a preference that lets decide what happens when you click on a link that wants to open in a new window. You can let it open an entirely new window, or you can choose to have it opened in a new tab instead.

In Safari, there is no such setting. At least not in the Preference. But you can still set up this behavior in Safari if you want. You do it by opening a terminal window and entering the following command

defaults write com.apple.Safari TargetedClicksCreateTabs -bool true

You may have to Quit and start Safari again for this change to take effect.