.cer abd .der are certificates and Firefox will try to install such a certificate in the Certificate Manager if such a file is send with the proper MIME type.

What is your intended use case for open this specific file type if you do not want to install these certificates or isn't that working as well ?

You can only add an action to "Options/Preferences -> General -> Applications" if the server sends such a file with a valid content type.

• https://support.mozilla.org/en-US/kb/change-firefox-behavior-when-open-file

i prefer view certificate as a temp and check parameters, but dont download and dont install to storage personal certificates(public key)

There are online certificate viewers where you can paste a base64 encoded certificate.

In Firefox you can inspect a certificate in the Servers tab of the Certificate Manager.

• Options/Preferences -> Privacy & Security
  Certificates -> View Certificates -> Servers

The Servers tab has an "Add Exception" button to open the "Add Security Exception" window. You can type/paste the domain in the location field and click "Get Certificate" to retrieve the certificate and click the "View" button to inspect the certificate.