Not sure why you got this. I have logged out and then I logged in again. No problem with the FF security system. Seeing the alert does not necessarily mean that the website you're visiting is trying to trick you into believing it is a different website - it means that Firefox isn't able to verify the identity of the website.You may have enabled SSL scanning in your security software such as Avast, Bitdefender, ESET or Kaspersky. Also check the date and time in the clock on your computer. Let's see if there are other guys who can encounter the trouble.
However I would follow that firstly...
You can retrieve the certificate and check details like who issued certificates and expiration dates of certificates.
- Click the link at the bottom of the error page: "I Understand the Risks"
Let Firefox retrieve the certificate: "Add Exception" -> "Get Certificate".
- Click the "View..." button and inspect the certificate and check who is the issuer of the certificate.
You can see more Details like intermediate certificates that are used in the Details pane.
If "I Understand the Risks" is missing then this page may be opened in an (i)frame and in that case try the right-click context menu and use "This Frame: Open Frame in New Tab".
Note that some firewalls monitor (secure) connections and that programs like Sendori or Fiddler (FiddlerRoot) can intercept connections and send their own certificate instead of the website's certificate.
The issurer may indicate the problem with your net provider' who blocks "adult" sites and other harmfull sites.