7. Support by newsfeed readers
Firefox 1.5 has support for live bookmarks for all of the above four formats. The previous release (Firefox 1.0.7) had no support for Atom 1.0. Plans for Firefox 2.0 and 3.0 are outlined in http://wiki.mozilla.org/Feed_Handling
Although Opera 8 has support for RSS and Atom 0.3, it has no support for Atom 1.0. Support for Atom 1.0 is available in the latest development release of Opera (Opera 9.0 Preview 1, released on October 20th 2005). I have done some experiments using Opera 9.0 Preview 1.