How do I get PKCS#12 key/certificate

Struggling with this signing process… It really shouldn’t be so hard…

All I’ve been given by Comodo for my code signing cert is a XXXX.p7s file

I’m trying to sign a XPI plugin and all the instructions require a PKCS#12.

Are there any good descriptions of how to get one.

On my Mac, the only export options are .cer and .pem (.p12 is grayed out - what does that mean?)