As you are using Selenium 3.8.0 you have to use GeckoDriver as a mandatory. But again as you are using Firefox v46.0 you have to set the capability marionette as False
through DesiredCapabilities()
as follows :
from selenium import webdriver
from selenium.webdriver.common.desired_capabilities import DesiredCapabilities
cap = DesiredCapabilities().FIREFOX
cap["marionette"] = False
browser = webdriver.Firefox(capabilities=cap, executable_path="C:\\path\\to\\geckodriver.exe")
browser.get('http://google.com/')
browser.quit()