Rich Internet Application columnist for ZDNET Ryan Stewart broke the news today that the WPF team are working on XBAP applications for Mozilla Firefox for the upcoming .NET Framework 3.5 (codename "orcas") release.
XBAPs (Xaml Browser APplications) are a special kind of ClickOnce deployed WPF application that is hosted in the browser inside a security sandbox, and currently only work in IE6/7. XBAPs integrate well into existing web-based applications providing a fairly seamless transition from regular HTML content to WPF application, since they are hosted inside the main browser window. Presumably this hosting is what makes getting XBAPs to work in Firefox more difficult than regular ClickOnce applications, which currently can work. The Firefox support would only be for versions of Firefox capable of running WPF itself (currently XP SP2, Server 2K3 and Vista, but may change for 3.5).
This also raises the question of .NET Framework 3.5 adoption - .NET 3.0 gets a hand-up in the adoption stakes because it is included in Vista, so it can ride the Vista adoption curve. Unless Microsoft make .NET framework 3.5 an automatic update for Vista its adoption will significantly lag .NET 3.0 for some time.