Ever since switching to Thunderbird, I’m happy… for the most part. There is this error where it cannot open or download attachments correctly. People complained and were told to upgrade. Guess what: no such luck!
Ton said that the second answer there might be correct.That is:
Go to Preferences > Advanced > General > Config Editor, and change the value of mail.server.default.fetch_by_chunks to false.
I did, could unfortunately not test conclusively yet (existing PDF attachments may be locally stored in corrupted format, not sure how to undo that).
If I find the answer, I’ll update the post.
Edit (15 sep 2011): Still no conclusive test, but I tested it on a new TB install (setting the values before adding my mailaccount), and the attachments that were previously borked now work. Which is sufficient evidence for me :)