install.rdf

<?xml version="1.0"?>

<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
     xmlns:em="http://www.mozilla.org/2004/em-rdf#">

  <Description
    about="urn:mozilla:install-manifest"

    em:name="Blocked.org"
    em:description="Blocked.org button"
    em:creator="My name"

    em:id="check@blocked.org"
    em:version="1.0"
    em:homepageURL="http://blocked.org"

    em:iconURL="chrome://custombutton/content/icon.png" >

    <em:targetApplication><!-- Firefox -->
      <Description
        em:id="{ec8030f7-c20a-464f-9b0e-13a3a9e97384}"
        em:minVersion="1.4"
        em:maxVersion="99" />
    </em:targetApplication>

    <em:targetApplication><!-- Thunderbird -->
      <Description
        em:id="{3550f703-e582-4d05-9a08-453d09bdfdc6}"
        em:minVersion="1.4"
        em:maxVersion="99" />
    </em:targetApplication>

    <em:targetApplication><!-- Sunbird -->
      <Description
        em:id="{718e30fb-e89b-41dd-9da7-e25a45638b28}"
        em:minVersion="0.2.9"
        em:maxVersion="99" />
    </em:targetApplication>

    <em:file>
      <Description
        about="urn:mozilla:extension:custombutton"
        em:package="content/custombutton/" />
    </em:file>

  </Description>

</RDF>