When I open https://stackexchange.com/, the padlock icon in my Firefox has a yellow warning triangle icon, clicking on it, the message says:
Connection is Not Secure
Parts of this page are not secure (such as images).
F12 console contains many messages like (all of them about gravatar.com URLs):
Loading mixed (insecure) display content “http://www.gravatar.com/avatar/195aa8870c17b0e8ad0c623af6f6466c?s=16&d=identicon&r=PG&s=30” on a secure page
Since the HTTPs version of that URL seems to work fine, I think they should be changed.
