Mozilla can display several web pages at a time in one window by using tabbed browsing. This is often more convenient than opening a new browser window for each new document. To display a document (open a link) under a new tab, right-click the corresponding link then select from the context menu. The different documents are now directly available by clicking the different tabs arranged at the top of the main window.
As soon as you start using tabs, an extra button appears to the left of the tab bar, which allows you to open a new empty tab. Press Ctrl-T to achieve the same. To close the currently active tab, click the button with the X to the very right of the tab bar. Right-click a tab to open context menu that offers several related actions, such as closing the tab or closing all other tabs.