The preview is shown because |wccr.getAutoHandler(TYPE_MAYBE_FEED)| returns null; I'm not 100% sure why yet: http://lxr.mozilla.org/mozilla/source/browser/components/feeds/src/FeedConverter.js#197 Consequently, we fall through to the preview page code further down the method. I'm not entirely sure what the right fix is here, but it probably involves WebContentConverter.js hacking. This is present on both branch and trunk.
This happens both with and without patches from bug 344651, FYI.
Hmm, so now I'm not reproducing it -- investigating...
Maybe this was just a trunk issue -- can still reproduce it there.
The _init method on WebContentConverter is never being called, so the hash of autohandlers is never populated. Somehow, it looks like the profile-after-change observer notification either isn't being received or isn't being fired.
I fixed this a while ago.