Looks like you have some bad certificates in your Firefox store. Here is the certificates from yahoo.com and google.com for my browser. You'll see that they are from a trusted certificate authority -- VeriSign and GeoTrust, where yours is from SHC Issuing CA.
The only other thing that might be happening is SSL inspection if your computer is on a corporate network and they have some kind of appliance that does SSL Inspection. But this is unlikely since it works fine with IE and Chrome.
Have a look at this article from Firefox on how to fix the issue.
I would be trying to delete the cert8.db file:
The file cert8.db in your profile folder may have become corrupted. Delete this file while Firefox is closed.
Open your profile folder:
Click the menu button, click help and select Troubleshooting Information. The Troubleshooting Information tab will open.
Under the Application Basics section, click on Show Folder. A window with your profile files will open.
Note: If you are unable to open or use Firefox, follow the instructions in Finding your profile without opening Firefox.
Click the menu button and then click Exit.
Click on the file named cert8.db.
cert8.db will be recreated when you restart Firefox. This is normal.
Failing that I would be uninstalling and reinstalling Firefox.
Also do a virus scan to check if something bad has infected your browser, just for piece of mind.