<hr /><i>dmitry added on 2011-08-25 12:36:36 UTC:</i><br />
I cannot reproduce exactly this, but Thunderbird shows me an error "feed uses invalid security certificate" for self-signed certs: http://i.imgur.com/FIEQ8.png. 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 https://www.fossil-scm.org/index.html/timeline.rss 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 “www.fossil-scm.org” which could put your confidential information at risk.".

However, feeds delivered via HTTPS with CA-signed certificates work well (example: https://codingrobots.org/qlfossil/timeline.rss).

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