Yup it is here - bug 253490 - Turn on the new RSS extensions and add it to the installer.
It isn't completely working yet, but it is ready for testing. Works very much like the Forumzilla extension. It even displays the webpage in full as specified in the <link> tags in the RSS feed like Forumzilla does, which isn't ideal. Atom feeds work fine though - only the <content> is shown in plain text format.
Well, on to the screenshots. I'm not in the mood for too much writing today.
Creating a new RSS account:
Management pane for an RSS account:
RSS account options:
RSS feed management dialog:
Adding a new RSS feed:
Message pane listing items in a RSS feed:
Grab the latest Thunderbird nightly build for some first-hand action.