I'm a big fan of Mozilla FireFox, and use it every day. I first heard about Mozilla Prism (formerly WebRunner) via Scott Barnes, and was immediately struck by how similar it sounded to a feature in IE since circa 1996 - HTML Applications or HTA's. Clearly I'm not the only one who's seen the simmilarity. HTA's give you full access to local resources like disk, allow you to customize the icon and disable the browser chrome. One of the touted features of Prism is that it runs as a separate process, so if your browser crashes it won't take your applications with it. This “feature” really only exists to work around an implementation detail in FireFox - that all FireFox windows are owned by a single process. Not all browsers use this approach, and thus don't suffer from this problem. The other question this raises in my mind is where does this leave Adobe AIR? If two of the major browsers support building desktop-integrated web applications using web technologies like HTML, CSS and JavaScript where does that leave AIR? I haven't really looked in-depth at all three offerings, it's possible that a small difference in one of them could be a deal maker or breaker for creating great applications. For example XmlHttpRequest seems like a very small feature but its widespread implementation (along with more mature CSS and JavaScript implementations) has allowed the browser to really take off as a richer platform for applications via AJAX.
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