Problem: Firefox suddenly barfs "Your connection is not secure" messages on major sites such as Mozilla, Google, Blogger, PIA, and possibly several others. But several other major sites (news, Bing, etc.) are OK.
Phones, tablets and other PCs on the same network are not affected. And there is nothing in recent news that Google has somehow messed up their security certificates and basically screwed the entire online population of the planet.
Solution: Chances are your PC clock is wrong, either because you just reset your BIOS or replaced the backup battery in your PC, and forgot to re-set your clock. If your PC thinks it's 2004, it will fail a lot of current security certificates.
Another possibility is your antivirus is interfering with Firefox security validations. This is not likely unless your AV and/or Firefox has changed recently, but is supposedly a known problem with Avast in particular.
Similar issues can cause a "Your connection isn't private" error in Chrome. For a list of possible fixes to both issues, see this post at Quora.