I cannot reproduce exactly this, but Thunderbird shows me an error "feed uses invalid security certificate" for self-signed certs: Such certificates should be used with caution, and rejecting self-signed feeds by default is a good thing to do for average users.

If I add the feed into OS X Mail, it does the same: "The certificate for this server is invalid. You might be connecting to a server that is pretending to be which could put your confidential information at risk.".

However, feeds delivered via HTTPS with CA-signed certificates work well (example:

I suggest you to get a CA-signed certificate gives them for free -- I use their certificate on my website without problems), or find a way to enable self-signed certs in Thunderbird.