If I get rid of the Adobe Reader plugin completely, so that mozplugger is the
plugin responsible for viewing Adobe Reader files, then when I visit such a
file, mozplugger correctly brings it up in Evince.

However, when I close the window, a weird window is left open behind it, with no
content in it.  This window is clearly owned by firefox, since when I quit
firefox, the window goes away.

This is probably related to bug 456016, but I'm not convinced it's completely a
duplicate, since the window that gets left behind after viewing with evince has
window decorations, and the window that gets left behind after viewing with
nppdf.so doesn't.

Also, if you click the X to close the strange phantom window, and then try to
visit another PDF file, firefox crashes.