Check the date and time and time zone in the clock on your computer: (double) click the clock icon on the Windows Taskbar.

Check out why the site is untrusted and click "Technical Details" to expand this section. If the certificate is not trusted because no issuer chain was provided (sec_error_unknown_issuer) then see if you can install this intermediate certificate from another source.

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 FiddlerRoot can intercept connections and send their own certificate instead of the website's certificate. Note that it is not recommended to add a permanent exception in cases like this, so only use it to inspect the certificate.

I added the error code that facebook is giving me on my first comment.

(Error code: sec_error_unknown_issuer)

So I dont have the option of allowing the certificate as an exception. I opened facebook in windows without a problem, and tried to follow the directions you gave, but I could not find the area where to find the certificate to copy it on firefox. The only certificate I could find is https://www.facebook.com/ i tried to add it to firefox but still it does not allow facebook More options, and please remember I am not that computer savvy so please use directions that are simple for me to follow. thanks

You should not need to make exceptions for mainstream websites that know how to manage security. Is Facebook the only site that has given you an untrusted connection error, or is it becoming a pattern?

If there is a pattern to it, a common reason is that Firefox is not fully set up to work with your security software, such as Avast, AVG, BitDefender, ESET, or Kaspersky, or another filtering program such as parental control software. Do you use any of those?