convert .p12 to .pfx

I am new with Comodo/certificates. I am using Firefox and my certificate is a .p12 file. I am using Installshield 2014 and it requires .pfx file to sign files. How can I get .pfx file ? Just rename .p12 to .pfx ? I tried to get the certifcate using IE, which sets a .pfx file but IE does not allow me to do that and generate an error.

Any idea ?
Thanks !