You should check the Folium version you are using. 0.2.0 has changed how popups are added, as explained in this issue thread: https://github.com/python-visualization/folium/issues/360
Popups now no longer take html automatically, but rather one must pass in a folium.element.IFrame of the html.
This notebook shows the differences in folium 0.2.0:
And the code would look something like this (found in the notebook above):
m = folium.Map([43,-100], zoom_start=4)
<h1> This is a big popup</h1><br>
With a few lines of code...
from numpy import *<br>
iframe = folium.element.IFrame(html=html, width=500, height=300)
popup = folium.Popup(iframe, max_width=2650)
NB: the way to create markers, therefore, is no longer simple_marker or circle_marker, but rather folium.Marker or folium.CircleMarker().
The docs have also been (recently?) put up.