At an event today on Microsoft's campus, Ian Ellison-Tayler, the WPF Product Unit Manager mentioned that XBAP support is coming to Mozilla Firefox as part of the .NET Framework 3.5 update that will ship with Visual Studio "Orcas". I've covered XBAPs in the past, but for those who aren't familiar, XBAP stands for XAML Browser Application and they are essentially Windows Presentation Foundation applications that are deployed as browser applications. Up to now, they've only been supported with IE7 but this news means they'll be available to a wider audience.
I think this is huge for RIA developers. While most of you know my affinity for rich internet applications on the desktop, I realize that the browser is still a platform of choice for many people. It's convenient and universal. For most of the people who see the browser that way, Firefox is really the only game in town. So while you could create an XBAP application and have full WPF functionality in your browser, most people wouldn't use it. Now however, the WPF browser experience can be used in both Firefox and IE7 which makes the potential audience for that type of deployment much greater. It makes WPF a very rich browser technology and a viable Rich Internet Application solution for Windows users.
Therein lies the kicker. The .NET 3.0 runtime is huge, and the .NET 3.5 runtime (which is what will support XBAPs in Firefox) will be even larger. It comes installed with Vista, and while Vista seems to be doing well, there are still a lot of Windows XP machines that won't have the .NET runtime installed. Trying to deploy a browser application that requires a 50 meg download on top of the actual application isn't viable. The .NET runtime just doesn't have the reach right now, but Windows is still a huge chunk of the market, and as Vista penetration grows and/or more people bite the bullet and download the .NET Framework, XBAPs may become a powerful way to deliver RIAs in the browser.