Click on View certificate to examine the current certificate
Firefox view certificate details
Then click on PEM (cert) to download and the certificate on your computer
Firefox download certificate
Click on the Not secure badge
Click on Certificate details
Chrome accept security warning details
Click on the Details tab
Then click on the Export... button then save the certificate on your computer
Chrome accept security warning details
Unfortunately, there is not any simple way to retrieve trusted self-signed file.
Please refer to Apple Safari documentation to download the .pem file stored on your system.