When can we see a "signed" FireFox plugin ?

I have been using FireFox for a while and Installed fair number add-ins too. I felt the plugins are creating a strong backbone for FireFox.

But….

I had never seen a single plugin with a proper digital certificate which provides the authenticity of the source. I always get to see the plugin installation as shown below..(note : FasterFox is one of the famous plugins)…

When we can see a “Signed” FireFox plugin ?