Firefox 4 Toolbar Overlay

I am trying to create my own Firefox toolbar and I have an overlay problem. So I already set up my environment (I can see my extension in the FF extensions manager), now I'm trying to add a new overlay.

chrome.manifest

content sandbox chrome/content/
overlay chrome://browser/content/browser.xul chrome://sandbox/content/overlay.xul

overlay.xul

<?xml version="1.0"?>

<overlay
    xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

<toolbox id="browser-toolbox">
  <toolbar id="findfile_toolbar">
    <label control="findfile_filename" value="Search for files named:"/>
    <textbox id="findfile_filename"/>
    <label control="findfile_dir" value="Directory:"/>
    <textbox id="findfile_dir"/>
    <button label="Browse..."/>
  </toolbar>
</toolbox>

</overlay>

When I restart my FF, I don’t see the result, where is the problem?