I would like to make a screen capture for an app (Firefox), but I am using a macbook, so the display is at 1280 x 800.

I think if I can somehow resize the window to 1920 x 1200, then I can use

Command + Shift + 4  and then space bar, and then mouse click on that app

to capture the whole app's window (which will be bigger than the screen)

So I can resize the window horizontally to 1920 by dragging the window to the left, and then make it wider by going to the bottom-right corner of the window.

But there seems to be no way to make it taller...

even

javascript: self.resizeTo(screen.availWidth+300,screen.availHeight+300);

on the Firefox address bar (URL bar) won't work... it can make the window wider than the screen, but not taller.

Is there any method at all?