The error you see is "SEC_ERROR_OCSP_INVALID_SIGNING_CERT" and looks like this:
If you prefer to continue using Firefox, you can modify the setting for certificate verification with the following steps:
- In the address bar, type about:config and press Enter.
- Click the I accept the risk button.
- Scroll down to security.ssl.enable_ocsp_stapling and double-click it to set the value to false.