Making a pop-up menu reflect a preference, even the default value

I'm trying to make a pop-up menu that reflects a preference.  One case
doesn't work though: When the menu is shown and the value of the
preference is at the default value, the  pop-up menu shows an empty
time instead of initializing to the default item being the one shown.

If the preference is set to a non-default value whether by the menu
itself previously or via about:config, the menu is properly
initialized to show the item that reflects the non-default preference
value.

What do I need to do to make the default case work?

The code I have looks like this:

      <preference id="intl.charset.fallback.override"
name="intl.charset.fallback.override" type="wstring"/>

...

        <menulist id="DefaultCharsetList"
preference="intl.charset.fallback.override">
          <menupopup>
            <menuitem label="&languages.customize.Fallback.auto;"
  value="auto"/>
            <menuitem label="&languages.customize.Fallback.arabic;"
  value="windows-1256"/>
            <menuitem label="&languages.customize.Fallback.baltic;"
  value="windows-1257"/>
            <menuitem label="&languages.customize.Fallback.ceiso;"
  value="ISO-8859-2"/>
            <menuitem label="&languages.customize.Fallback.cewindows;"
  value="windows-1250"/>
            <menuitem
label="&languages.customize.Fallback.simplified;"  value="gbk"/>
            <menuitem
label="&languages.customize.Fallback.traditional;" value="Big5"/>
            <menuitem label="&languages.customize.Fallback.cyrillic;"
  value="windows-1251"/>
            <menuitem label="&languages.customize.Fallback.greek;"
  value="ISO-8859-7"/>
            <menuitem label="&languages.customize.Fallback.hebrew;"
  value="windows-1255"/>
            <menuitem label="&languages.customize.Fallback.japanese;"
  value="Shift_JIS"/>
            <menuitem label="&languages.customize.Fallback.korean;"
  value="EUC-KR"/>
            <menuitem label="&languages.customize.Fallback.thai;"
  value="windows-874"/>
            <menuitem label="&languages.customize.Fallback.turkish;"
  value="windows-1254"/>
            <menuitem
label="&languages.customize.Fallback.vietnamese;"
value="windows-1258"/>
            <menuitem label="&languages.customize.Fallback.other;"
  value="windows-1252"/>
          </menupopup>
        </menulist>


-- 
Henri Sivonen
[email protected]
http://hsivonen.iki.fi/