How do I make "startx iceweasel" launch full-screen?

I've done a clean install of Raspian Lite on my Rasperry Pi (there is no desktop environment installed). I'd like a kiosk-mode browser running fullscreen without having to worry about maintaining/securing a DE. After installing Raspbian Lite, I did apt-get install iceweasel and apt-get install x-window-system. I'm now able to login and run startx iceweasel and I get a functional iceweasel as required; however it's only about a quarter of the screen! The console I ran startx from was using the full screen; but after launching X/iceweasel the used area got smaller. Ideally I'd like it to inherit the resolution the console was already using, but I'd settle for having to provide them manually if that's not possible.