Convert Thunderbird Subscriptions to OPML

From Kevin Hemenway, O'Reilly Network:

With the release of Thunderbird 1.0, there's still the missing ability to export OPML files of your RSS subscriptions. After the jump, I'll explain how to generate your own OPML file based on Thunderbird's feeds.rdf, as well as how to alphabetically sort your subscription list. Read more from this post.