If you had used Office products there is a classical Split menu option which gives you multiple views of the same window. Very useful when you want to compare two documents or two parts of same document. The same functionality can be achieved in Firefox with Fox Splitter Add-on.
This add-on adds split menu button which you can use to split a page above, right, left and down. This can be repeated with the split windows again if you want. The address bar is not available with the split menu and there is no tabs so you cannot open another page there but you the address bar is available for the main window where you can open another website and split it again.
This add-on comes very handy for web site designers who want to check multiple websites in one single window instead of using tabs and for people who do research for comparing plain text websites etc. Download Fox Splitter