Split Browser is an extension that will split the content area of a tab as you desire. It will help you in various cases. For example, to compare multiple Web pages, to show a calendar always, and so on. You can split browsers over and over. Status of split browsers are saved, and automatically restored on the next startup.
Windows 95/98/Me/NT/2000/XP/2003 Server/Vista, Firefox 3.0 - 3.7a1pre