So far, in Firefox 19 (Linux) this extension is working perfectly well without slowing my 9-year-old computer down. Being fully resizeable and having separate navigation buttons and a basic menu for saving, bookmarking or changing the location is a useful touch.
It would've been preferable if the button defaulted to using an icon instead of text; I wouldn't care, except changing it requires the browser be restarted. More problematically, the options window is too small to include the text & buttons, and doesn't allow resizing. If my desktop environment (KDE 4) didn't let me force windows to change size, I wouldn't have been able to hit the buttons to save my settings.
PS. It seems from what other creators have done that it's possible to rename an extension, so please change it to "Split Panel" so it's spelled correctly! Or perhaps rename it something else entirely like "Window Panes" so it's not potentially confused with the rival that's getting an increasingly bad rep.