I would like to serve xbaps from the VS web dev server (cassini) to Firefox, but when served from the dev server, Firefox offers to download this file. As far as I can tell, this is because the dev server is serving the xbap file with a mime type of "application/octet-stream" instead of "application/x-ms-xbap", which works when served from IIS.
Does anyone know how to change the mime type which the dev server uses for *.xbap files?
you can't. WevDev.WebHost is fairly clumsy when issuing content-types and has a very limited range of specific content-types.
You can use CassiniDev. The latest release provides extended content-type support, including .xbap.
see http://cassinidev.codeplex.com/SourceControl/changeset/view/49870#894160 for a complete list of supported types.
Update: your problem may be that you installed FF after 3.5sp1 and do not have the NPWPF.dll in your FF plugins directory. Do you have this file?
Update 2 I have just released a version of CassiniDev that is a great drop in replacement for Visual Studio's Development server. It's enhancements include improved content-type support and integrated traffic logging/viewing.