Useful obscure firefox extension (Mac)

In Firefox on the Mac, whenever you press Cmd-N to open a new window, it tries to place the new window slightly below the current window (so you can see both titlebars), even if that puts the bottom of the new window off-screen (bug 218214). It only seems to happen with Firefox, and only on the Mac version. This is especially annoying since Apple's fondness for widescreen displays makes vertical space even more precious. It turns out there's a workaround: this extension (direct link to onscreen-0.1.xpi) will move all new windows as necessary to fit on the screen.