Open a new tab: Click the New Tab button to open the previous or next page in a new tab.
You can also type in the in the upper-left corner of the thumbnail.
Convert all Safari windows to tabs in a single window: Choose Window > Merge All Windows.
Avoid duplicating a tab: If you start to type a website address in the Smart Search field and you already have that address open in an existing tab, Switch to Tab shows the website. Choose the website to use the existing tab.