As we had discussed on the mailing list and on irc, use of
set_protocol_handler seems to trigger this problem.  We might as well move
the discussion here to the bug tracker..

To bring this problem on, do, for example:

  set_protocol_handler("mailto", "https://mail.google.com/mail/?extsrc=mailto&url=%s");

Then follow a link to some content type that is not internally supported
by Gecko.  PDF is a convenient test case.