The old window stays open in the background. Try not to let too many of these extra windows get opened. Close some of them occasionally. Too many windows open risks a browser crash. Netscape has several interesting commands that aren't advertised.
Clickable links can reopen any of the sites.
These commands are very useful when tracking student misuse. Take advantage of right-click menus. On a Macintosh, click and hold down until the menus pop up.