I've run into three closely related problems with
First, application types listed as "Unknown:Prompt User" in
the helper applications list don't seem to do this. (It
saves to disk instead.)
Second, when I specifically change the application, it
doesn't seem to "take" (is forgotten the next time I return
to the list).
Finally, because of these I can't seem to find any way to
get Communicator to show PDF files, rather than saving to
disk. I've verified (using wget -S) that the file type is
being reported as application/pdf by the servers.
I'm probably just doing something dumb, but I can no longer
figure out how to simply "ask a question" at this site, so
I've resorted to using Bugzilla... hope that's okay.
Hmm... the save dialog seems to be the default for
'prompt user'. Aside from that, choosing helper
applications seems to work fine here.