Firefox "Connection Untrusted"

My computer came with Windows 7.
Around this time last year I updated to Windows 8, and the 8.1.
Yesterday I removed 8 and went back to 7.
Now Firefox has a "Connection Untrusted" warning before going into most websites. REALLY annoying.
I've tried changing the clock on my computer like most other forums say to no avail.
I turned of Windows Firewall to see if that was the problem and it wasn't.
This is REALLY annoying. I've never had to deal with this before. Any suggestions?

This belongs in the Browsers & mail forum.

Start here, and note the link on its first line:

Error loading secure sites - MozillaZine Knowledge Base

I'm using Chrome right now. IE works also.
All of this didn't start happening until I removed windows 8.

if you had installed any software which effects your browser like avast antivirus which detects any malicious website ,which interfers with your browser then uninstall it if it is installed on a wrong date or time and reinstall it