Taking a whole page screenshot with Selenium Marionette in Python

After the recent Firefox upgrade to version 47 we were forced to install the Marionette extension to keep being able to use selenium webdriver, and in my case also upgrade selenium from 2.52 to 2.53.

I use the python version of selenium webdriver to acquire high resolution images of maps rendered in HTML and JavaScript. previously this worked fine in firefox and the screenshots could be taken of the whole page, far beyond the dimensions of my own screen. However with the recent changes the screenshot is taken only of the area visible on screen. I use the following code:

import time
from selenium import webdriver
from selenium.webdriver.common.desired_capabilities import DesiredCapabilities

caps = DesiredCapabilities.FIREFOX
caps["marionette"] = True

browser = webdriver.Firefox(capabilities=caps)
browser.get(html_file)
time.sleep(15)

browser.save_screenshot(image_name)
browser.quit()

I have already considered: downgrading, stitching together several screenshots or switching to Qgis. However I would prefer a more elegant solution which would allow me to keep using the latest version of firefox and roughly the same methodology. Does anyone know a solution to this? perhaps by tricking selenium in thinking the viewport is larger? or by using another linux supported browser which does allow for the full page screenshot?