Don't know if I agree with this, but it sure is provocative! What about Flash and Laszlo?
Back to web applications. Mozilla is now the best platform for doing such development. It has the best standards support, is cross-platform and has the best devloper tools: DOM Inspector and Venkman, and perhaps even mindshare. In fact (market share be damned), it makes sense to first write yout rich web application for Mozilla. Your code will be cleaner to start off, will be better structured and will most likely work in other browsers - but that's my opinion, others say target MSIE first, just look at marketshare! - I would rather ignore that and get the compelling app.