A better idea than a web interface IMO is Usenet access simply built into a browser by default. It makes sense to have Usenet in an email client, because they are closely related, but not many people use email clients any more, so not many people are going to find Usenet that way.
If Firefox had an option to open newsgroups, with proper filtering (not just on the overhead headers, but on the HEAD of each message), I would use it all the time, and I believe many thousands of other people would too.