I think I found a small FF2/Zimbra bug. If you have an email that has a another email as an attachment, and click to view the attached email (as a popup), then close the popup, then click the delete icon to delete the email, it is moved to the trash, but the UI doesn't update and still shows the email in the inbox. Clicking 'Get Mail' will refresh the UI and show the email was really moved to trash.
I tried this on both XP/FF2 and OS X (intel)/FF2 with same behavior.
Here is bug I opened.
http://bugzilla.zimbra.com/show_bug.cgi?id=11622