HTTPS page is redirected to HTTP

I am using Firefox 46.0.1. When I go to https://chriszarate.github.io/supergenpass/mobile/, everything behaves as I would expect.

When I visit the same URL in a private tab, I am redirected to http://www.supergenpass.com/mobile/ (no https).

In the non-private console, the only requests I see are to github.io. The private console shows a 301 redirect to supergenpass.com

I first noticed this yesterday using Firefox Developer Edition on a different machine (DE displays downgrading behavior, regular edition does not. Didn't think to try private window). What might be going on here?