A run of text that contains an ideographic space (U+3000) and no other ideographic characters will give a hex box for that character, since Pango considers spaces to be neutral with respect to font selection. This comes up in Mozilla+Pango because Mozilla tends to break the input text into smaller runs. http://bugzilla.gnome.org/show_bug.cgi?id=145275
Should be fixed in 1.6.0-2