Firefox has supported RSS since its very first version as live bookmarks and in the sidebar, in fact Microsoft asked Mozilla if they could use the Firefox RSS icon to help create some kind of "standard". If course, if you want a real reader then Kanebear is correct, Flock or even Thunderbird will be a much better choice. The mere fact that it took Microsoft till the last months of 2006 to embrace RSS proves they are waaaay too late for the party.

And of course, unlike IE7 in Firefox you can add one of 100's of RSS enhancing plugins.