which cert type?

Hi,

I have successfully tested signing my firefox extension using the
instructions here:
http://www.mozdevgroup.com/docs/pete/Signing-an-XPI.html

Now I want to buy a code siging certificate to deliver my extension
professionally. What sort do I need to get?
Netscape object signing I presume?
or
Java signing for jar files perhaps?

Can anyone offer any advice on what to purchase from where?

Thanks

Carey