Firefox 3 has a race condition that affects the way that <object is invoked, loaded and detected. What this means is that the forthcoming Firefox 3 release will break some Silverlight 1.0 installation experiences on web sites that used the original Silverlight 1.0 SDK Silverlight.js detection file.
The Silverlight 2 SDK has an updated Silverlight.js file that not only solves this problem for all versions of Silverlight but also eliminates browser restart that was previously needed for new installs.
You can get the SDK as part of the Install Silverlight Tools Beta 2 for Visual Studio 2008 or through the download center.
Silverlight Streaming will support hosting Silverlight 2 Beta 2 later this week.
If you want to link to the Silverlight.js file hosted at Silverlight Streaming (best practice) the one to use is http://agappdom.net/i/silverlight.js