The default mime type for avi files (video/avi) does not work in Firefox. The correct mime type is: application/x-mplayer2
You can set that in VB in the control panel on the left side: select Attachments > Attachment Manager > Edit avi > then for mime type, type in: Content-type: application/x-mplayer2
That should do it, you can now play avi files in both IE and Firefox