fixed DEBUG build for mozilla browsers

[Brian Fundakowski Feldman]

Note that I have only tested that this fixes the DEBUG option for the
firefox port, but I don't see why it wouldn't apply to the rest of
them.  The problem is more fully described and patch provided here:
<http://bugzilla.mozilla.org/show_bug.cgi?id=253080>

-- 
Brian Fundakowski Feldman                           \'[ FreeBSD ]''''''''''\
  <> green at FreeBSD.org                               \  The Power to Serve! \
 Opinions expressed are my own.                       \,,,,,,,,,,,,,,,,,,,,,,\