There appears to be an incompatibility issue with Firefox 8 and the Exchange 2010 Outlook Web Application.

When attempting to Save or Open attachments in emails viewed through Outlook Web in Firefox 8, the attachments attempt to Save or Open as a file called attachment.ashx.  The attachment is not broken, and you can “Save As” and rename the file and file extension to the correct name and it will work.  Our recommendation, at this time, is to use Internet Explorer when viewing Outlook through a web interface.