is there a way to split a single browser's window/tab into 2 viewing areas?

Solution 1:

You could try the Split Browser addon.

Solution 2:

If you have any experience with Javascript you could write a GreaseMonkey script for Firefox that pops the header row into a static (non-scrolling) area (div) on the screen.

You would need to get enough identifying information about the header row to be able to select it using Javascript - you could use the Firebug extension to do this.

Solution 3:

I'm not familiar with the windowing options of Firefox and IE, but I know how you could do this in Opera. I assume that similar capabilities are available in Firefox.

You could open the document in two separate tabs and then go to Window -> Arrange -> Tile Horizontally

Then, use the bottom window to scroll independently of the other.