I'm getting it too, I'm using Windows 10 Pro and Firefox. I think it has something to do with the site itself.
I clicked on the "advanced" button and got this message:
"Familysearch.org uses an invalid security certificate. The certificate expired on June 5, 2017 at 2:00 PM. The current time is June 5, 2017 at 3:18 PM. Error code: SEC_ERROR_EXPIRED_CERTIFICATE"
It is because the site starts with "https" apparently it is supposed to be secure, other sites like RC for example don't have the https in front of the "www"
There is apparently a way to bypass the block. If you click on the advanced button and see that message there is an "add exception" button, that will pop up a box with an option to override the block.