As I've mentioned, the new "Live Bookmarks" feature coming up in Firefox is not super useful. As in, it's not going to have the features to replace an aggregator. Which is fine. As I understand it, the purpose of adding it was to be able to get bookmarks from a remote location, even if it's only read-only for now. The idea, I guess, was to use RSS stuff, as it's an already widely-deployed format. So, in that light, it's at least pointing out to cool stuff in the future, though where the practical benefit will lie is still to be seen.
However, the RSS integration in Mozilla Thunderbird is ready to replace your favorite e-mail client style aggregator. Well, ok, not quite. It's in the nightly builds, and I'm sure there are bugs to be worked on. But it does support the standard concept: feeds as folders in your mail client, with posts as messages. Those wanting a full client aggregator that works on multiple platforms, Thunderbird may be your answer.