In the Web Search Tab or from the results that comes up when you click a leaf. (Same position in the program)

To get the webpage to open in the browser that is the default in the OS. Click the icon "Browser Options" and select "Open in New Window" See attachment

So if you have made Firefox the default browser for your system, when you click the "Open in new window" in the drop down menu you get when you click "Browser Options" on the search page, the page will be opened in a new Firefox window.