How is this different from right-clicking and selecting "Open link in new tab" without downloading Split Window?
Only one tab is visible at a time. With this add-on you could split the main browser window into two and view both pages simultaneously in one browser instance.
I often have multiple IE and FF instances open and sometimes more than one tab in each so I'm not sure I'd really use the split window feature, but then again if you wanted to compare 2, 3, or 4 sites at the same time it would be easier using a plugin in like this rather than sizing 3 or 4 browser windows and tiling them next to each other on the desktop.