In Firefox 7 beta the "fileName" property on a File object is missing. This will result in an error when using the HTML5 upload methods. The "name" property does exist and holds the required value. On line 231 of file "plupload.html5.js" it should say
files.push(new plupload.File(id, file.fileName || file.name, file.fileSize || file.size));
files.push(new plupload.File(id, file.fileName, file.fileSize || file.size));
to test for the existence of the fileName property.
This has been fixed in latest commits.