In some cases Zimbra desktop (Ubuntu 9.04) complains upon trying to open attached PDF files. The error message is part of a dialog that says "Download error" in title bar, and the displays
/tmp/name of file.pdf could not be opened, because the associated helper application does not exist. Change the association in your preferences.
I just upgraded to FF3.5, which may be the issue. Acrobat files open properly within FF3.5. Acrobat files also open properly from file system.
I have tried changing the settings in Firefox to "always ask" with no effect.