You have asked Firefox to connect securely to mail.google.com, but we can’t confirm that your connection is secure.
You’ve probably seen this error lots of times and wondered what it meant. Secure websites that require HTTPS connection such as Gmail, Paypal, Yahoo Mail, Facebook and others often show this error message. If you proceed and add exception, you might notice the page won’t load correctly and the display might look rather abnormal. In such cases, some images may not be displayed and lots of functions on the web page may be disabled.
I really don’t need to explain all the technicalities, I’m going to keep it as simple as possible. Some sites require your connection to be encrypted and secure so as to protect your data. Websites that require SSL (Secure Sockets Layer) encryption or HTTPS connection always send some sort of certificate to your web browser which it checks whether it’s valid or not. When the secure connection certificate is invalid, your browser may be unable to connect with the website. There may be two reasons for this:
1. You computer time is incorrect.
2. The site is using an expired certificate
3. The site you’re trying to connect to may be illegal, using a fake SSL certificate
How to fix this
If you’re pretty sure the site is legal and you shouldn’t be having problems with HTTPS connections, be sure your computer date is set correctly. This should get it fixed.
If your computer date and time is set correctly and you’re still seeing this, there’s definitely a problem with the website and you should be careful about sharing sensitive data with such sites. No legitimate site will use an invalid SSL certificate.