Hi hari3642, you should NEVER need to add an exception for a legitimate website. Have you seen error pages like this on other sites??

To further investigate why Firefox is not accepting the certificate, could you click the error code on that page and copy/paste the coded certificate text into a reply:

Error code: SEC_ERROR_UNKNOWN_ISSUER

(I know you asked about an exception but as explained on the error page, Firefox doesn't allow exceptions when sites specify "strict transport security.")

i am having this trouble only with google but i guess i have to use chrome. anyway thanks for ur reply

Okay, I asked if you could provide me information from the error page. Were able to find that, or have you already given up?

Error code: SEC_ERROR_UNKNOWN_ISSUER this is what it is showing as error

Okay, click the SEC_ERROR_UNKNOWN_ISSUER on the error page and a new panel should appear with the coded text of the untrusted certificate. Select that, copy it, then paste into a reply so I can analyze it. Thanks.

Maybe I am not explaining it well. Here's someone else's explanation:

Follow the steps below to collect the Certificate Error troubleshooting information.

Note: Please be aware that the certificate keychain may or may not be unique. If this is a self-signed certificate please take note to remove the information below "--BEGIN CERTIFICATE"

1. From the error page pictured below look for the error message.
   
2. Click on the "Advanced" button and it will show the error link.
   
3. Click on the error link and you will be taken to a page that you can copy the error text to the clipboard. The image below has blocked out the certificate for this example.
   

Other resources to compare errors: https://badssl.com/

https://support.mozilla.org/en-US/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER