Firefox says parts of the page are not https (most likely images). The site is the personal site of professor and statistician, Edward Tufte, known for having strong opinions about data visualization and the use of PowerPoint.

This seems to be the offending image: http://ianwojtowicz.com/Red%20Shift/Red-Shift-Earth-6400px.j... which is now a 404 anyway (or rather, it's a 301 followed by a 200 with text explaining it should be a 404).

yes