I find I have the same problem with Firefox. It doesn't work uploading files using the file browser. But when using IE8 uploading using the file browser works fine. The error message with Firefox is that the connection has been lost. Obviously a bug in the Beta that needs ironing out.
"The NASman - Storage for your digital future"
Synology Model: DS1511+, CS407e
Firmware: 1358, 1285
HDDs: 4 x Samsung HD204UI 2TB
HDDs: 4 x Samsung HD501LJ 500GB
DMAs: 1 x XBMC,1xWDTV Live