I've made a toolbar for firefox. The toolbar contains an item (menu).
As follow, see the code:

<menu id="partner-menu" label="Partner">
   <menupopup id="misc-popup">
      <menuitem label="Google.com"
oncommand="LoadURL('http://www.google.com/')" />
</menupopup>
</menu>

The Function "LoadURL" loads an URL in the Browser window:

function LoadURL(URL)
{
        window._content.document.location = URL;
        window.content.focus();