Opening attachments on my MacBook (Tiger) was a huge pain after switching to an Exchange email server. Double-clicking an attachment would launch the concerned application but would not download the entire file from the email server - while Outlook Web Access worked perfectly. The attachment files in the temporary folder would always be 24 KB. Finally a solution somebody suggested in a forum: there is an error in reporting attachment sizes, so make Thunderbird download the message 'entirely' instead of 'chunks'. It works! Back to some real work.
"Thunderbird may truncate the attachments. Exchange and Gmail are know to cause this problem.
Go to the Config Editor, and enter chunks in the filter box. Toggle mail.server.default.fetch_by_chunks to false. That will make Thunderbird ignore the incorrect information."