Initial skeleton of the extension

chrome.manifest

+content    moderation  chrome/content/
+overlay chrome://messenger/content/messenger.xul chrome://moderation/content/mylabel.xul

chrome/content/mylabel.xul

+<?xml version="1.0"?>
+<overlay id="sample"
+        xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+        <!-- <script type="application/javascript" src="chrome://moderation/content/overlay.js" /> -->
+        <statusbar id="status-bar">
+           <statusbarpanel id="my-panel" label="Date" />
+       </statusbar>
+   </overlay>

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:id>moderation@delpeuch.eu</em:id>
+        <em:version>1.0</em:version>
+        <em:type>2</em:type>
+
+        <em:targetApplication>
+            <Description>
+                <em:id>{3550f703-e582-4d05-9a08-453d09bdfdc6}</em:id>
+                <em:minVersion>1.5</em:minVersion>
+                <em:maxVersion>10.0.*</em:maxVersion>
+            </Description>
+        </em:targetApplication>
+
+        <em:name>Modération pour Thunderbird</em:name>
+        <em:creator>pintoch</em:creator>
+
+    </Description>
+</RDF>
+