some rss feeds don't show up as Livemarks when bookmarked...

[techguy]

So, when you bookmark some RSS feeds, the bookmark does not appear as a
Livemark - so you don't get the Livemark's submenu listing all the
individual items in the feed.

a couple non-working feeds are:
http://rss.clipmarks.com/
http://rss.furl.net/members/rssPopular.xml?days=1

one that works:
http://news.google.com/

the little feed icon doesn't show up for the broken feeds, of course,
but i'm not concerned with that - i presume that's just a simple fix -
having whatever website add the appropriate <link...> tag.

I looked at the above broken feeds and a few others that don't present
themselves as Livemarks when bookmarked in Firefox 1.5 (Windows XP
Home), but I couldn't see anything obviously wrong. I've loaded those
above URLs into the Google Labs Reader and it works no problem, but
what would cause Firefox to not recognize them as Livemarks?

One 'broken' feed I noticed (i think Flickr) had an opening tag of
'<feed...>' instead of '<rss...>', but that was the only major
difference I saw, and most of the broken links start with the typical
'<rss...>' tag.

I'm working on some Livemark-related extensions, that's why the
question...