.p12 is an alternate extension for what is generally referred to as a "PFX file", it's the combined format that holds the private key and certificate and is the format most modern signing utilities use. If you have a .p12 file that you exported from Firefox or Safari just rename the .p12 extension to .PFX if you need to, it's the same format.

If your signing tools refer to a PKCS12 file, that is the same thing as well.