Fox Splitter inexplicably has the description belonging to an earlier project by the same name. *This* extension clones the Firefox window so there's two of them -- the old one, which the description is all about, really does let the user show more than one website within a single window.
To be fair, I'm going to review the extension as if the description didn't exist or was marginally accurate.
It works fairly well for what it's intended to do (create two synchronized windows), and has all of the options that a power user could hope for; I also really appreciated that it's a No-Restart install.
I did run into some problems, using Firefox 19 for Linux:
-- The location bar on the newer window was halfway obscured by the webpage; nothing I tried would fix it.
-- The toolbar-hiding settings couldn't apply to the add-on bar, as it's coded for the defunct statusbar.
-- It was impossible to align the windows without putting them both against the bottom of the screen; one would automatically jump a little higher/lower than the one I moved.
-- When I resized a window and its twin did so as well, they moved but still ended up overlapping quite a bit.
If I was doing the right kinds of online research right now, I'd find this extension very handy, so I'm keeping it bookmarked. Given the programmer's seeming inability to update things to match changes in Firefox (or his/her own extension) I assume "Fox Splitter" won't exist by then. :-/