In this blog post, Joshua Holt discusses how to debug Silverlight in the new Firefox version.
Today Mozilla released FireFox 3.6.4, and in the new version plugins are isolated in their own process. As such, the VS debugger attaches to “firefox.exe” instead of the new process FireFox creates named “plugin-container.exe” when you start debugging a silverlight application. There are 2 fairly easy fixes.