When using the integrated search in Firefox normally the result is shown in the browser tab that is currently shown. In my opinion that is quite nasty for one reason: In case you want to search something that does not correspond with the content your current tab displays, the result replaces the content.
But of course there is a way to change it. At first enter about:config into the address bar. Then search for browser.search.openintab. After that set the value to true. Now every time you search using the search bar (e.g. by using the shortcut Ctrl+K) the result is displayed in a new browser tab.
But be careful! Using the about:config is dangerous. You can damage Firefox in a way you cannot repair it after that.