Next Mail and News. All my newsgroups had been converted so it was easy to test them. The first thing I missed was support for the space bar. Almost all the newsreaders change article by pressing space. It would be nice if Mozilla would too.
Cursor keys work well, though.
After reading some news I decided to subscribe to a new group. This took over five minutes and consumed about 100 MB of memory. The newsserver I use has a huge amount of newsgroups and managing a big list like this was very slow. As Mozilla was using 167 MB of memory at this point, I decided to restart it. Even though the newsreader needs a lot of work, this build gave me a good impression and it's definitely a keeper.