Firefox 4 is near the end of its beta cycle but what is so special about this release? Why not see for yourself on the new demo site, the Web of Wonder (requires Firefox 4 beta but some demos do work in Chrome and Safari). I'll be honest, as a web developer, the new power of HTML5, CSS3, SVG, WebGL, etc totally blows my mind. If you're not a web developer then, well, it might not be as impressive.
This is our future, and it's a future without Flash. Feed your imagination! The best way to understand what's going on is to watch the screencast before playing around. There are a lot of subtle things you might miss, things that are not possible with older web technology. The Remixing Reality demo is the most interesting one of the bunch. It uses JavaScript to analyze and complement a WebM video by adding a WebGL layer as the video plays.
There are still a few demos cooking up that haven't been released yet due to bugs. Keep your eyes peeled. You can also hack on the demos yourself over at github.