This may be a problem with my environment, but I thought I'd throw it out there anyway.
I've started getting the follwing message screens on several sites - Dropbox and Google+ to name two.
Your connection is not secure
The owner of www.dropbox.com has configured their website improperly. To protect your information from being stolen, Firefox has not connected to this website.
This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox may only connect to it securely. As a result, it is not possible to add an exception for this certificate.
Any idea where to begin looking? I've got regular Firefox 49.0.2 installed in other virtual machines and they work just fine.
Thanks in advance.