The XBL prefwindow bindings are removed in Firefox 60, so this adds them
back, along with necessary styling, to allow the existing preferences to
work until the preferences are rewritten with React. The preferences.xml
file in the Mozilla source has platform ifdefs, but since this is a
temporary hack I've just duplicated the file for each platform with the
necessary lines included.
I haven't yet tested the styling on Windows/Linux.
