Digital signatures for macros

Hello,

I've been trying to work out (1) what type of certificate you need for
signing macros and (2) how to get the option to do that.

I've got both a document signing certificate and a code signing certificate
and, after installing them in thunderbird, they both appear for document
signing. For macros, however, I can't seem to make the macros > digital
signatures option enable.

I've enabled experimental features, which gave me macro recording, and threw
in a hello world macro to make sure there was something to sign but digital
signatures are still greyed out.

I'm sure there's an answer but lots of googling and both irc channels
haven't provided one. Can anybody on the list help?

Currently running Version 4.0.4.2 (Build ID: 4.0.4.2-9.el6) on CentOS. Any
help would be much appreciated