chrome:
 - content\
   -- contextsearch.css
 - skin\
 - chrome.manifest
 - install.rdf
xul:
<?xmlzstylesheet href="chrome://contextsearch/content/contextsearch.css" type="text/css"?>  
...
<menuitem id="my-menu-item"
                  insertafter="context-selectall"
                  accesskey="J"
                  class="menuitem-iconic"
                  label="tests"
                  oncommand="test(event)"/>
...

css:
#my-menu-item {
    list-style-image: url("chrome://contextsearch/content/skin/icon24.png");