To preview a tab, move your mouse pointer over the tab in the Tab Bar. A visual preview of the tab’s contents will appear below. This feature can be enabled/disabled in Settings > Tabs > Tab Display > Tab Options > Show Popup Thumbnails.
Toggle permanent tab previews by dragging the bottom edge of the tab bar down or up, as shown below. This will reveal or hide thumbnails for all your open tabs. To quickly open this view, simply double click on the border rather than dragging. This feature can also be turned on/off in Settings > Tabs > Tab Display > Tab Options > Show Tab Thumbnails.