We've found that double-clicking on mail attachments in Thunderbird doesn't work under Gnome, though it does under KDE. But there is a simple workaround and also a fix - a wacky one, but an easy one.
right-click on the attachment and choose Save As ...
open the Nautilus file browser (Places, Home Folder) and double-click the file
log in with KDE (go to the Session menu on the login screen)
open any attachment by double-clicking
log back with Gnome
Double-clicking attachments should now work. I'm sure that there's some reasonable explanation, but I think I can live with the mystery for now.