Firefox forces secure connections, but won't load them.

I have a new computer, with a new installation of Firefox that is failing to load numerous pages. It won't load anything over a secure connection. Sometimes I get a general "secure page cannot be loaded" sort of error and sometimes I get an error that specifies a problem with the certificate. So far, every one I've seen says the certificate is not yet valid (it has a future date), but I can't swear that's the only one I'd get on any site. Since Firefox is also FORCING certain sites (*cough* Google *cough*) to load securely, this is crippling my ability to do what I need to do. The computer came with McAfee installed. I uninstalled it. As best I can tell, there is no longer security/virus protection software of any kind on this computer. It's Windows 10, and whatever is the newest version of Firefox. This is a bran-new download/installation of Firefox; I have not installed any add-ons, imported any settings, etc. It's completely clean and "out of the box," as it were. The proxy setting is set to default, and I don't see anything in the security settings that looks like it should be causing this issue.