Initial (working) commit

chrome.manifest

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+content     passworddialogtitle    content/
+overlay     chrome://global/content/commonDialog.xul    chrome://passworddialogtitle/content/settitle.xul

content/settitle.js

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+window.addEventListener("load", function windowLoader(e) {
+    this.window.removeEventListener("load", windowLoader, false);   // Only need it to run this _one_ time
+    startup()
+}, false);
+
+function startup() {
+    let displayString = "Enter password: %MAIL%";
+    let description = document.getElementById("infoBody").innerHTML;
+    let reMail = /(?:[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*|"(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21\x23-\x5b\x5d-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])*")@(?:(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?|\[(?:(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?)\.){3}(?:25[0-5]|2[0-4][0-9]|[01]?[0-9][0-9]?|[a-z0-9-]*[a-z0-9]:(?:[\x01-\x08\x0b\x0c\x0e-\x1f\x21-\x5a\x53-\x7f]|\\[\x01-\x09\x0b\x0c\x0e-\x7f])+)\])/i;
+    let mails = reMail.exec(description)
+    if (mails != null && mails.length == 1) {
+        let mail = mails[0]        
+        let host = description.substr(description.lastIndexOf(' ')+1, description.length-description.lastIndexOf(' ')-2)
+        document.title = displayString.replace("%MAIL%", mail).replace("%HOST%", host)
+    } else {
+        console.log("password dialog title: Mail RegEx didn't match description: ", description)
+    }
+}
+

content/settitle.xul

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+<?xml version="1.0"?>
+<overlay id="passworddialogtitle-settitle" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
+    <script type="application/javascript" src="chrome://passworddialogtitle/content/settitle.js"/>
+</overlay>

install.rdf

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+<?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>passworddialogtitle@jonny007-mkd.de</em:id>
+    <em:name>Password Dialog Title</em:name>
+    <em:version>0.13</em:version>
+    <em:creator>Jonny007-MKD</em:creator>
+
+    <em:targetApplication>
+      <Description>
+        <em:id>{3550f703-e582-4d05-9a08-453d09bdfdc6}</em:id>
+        <em:minVersion>60</em:minVersion>
+        <em:maxVersion>61</em:maxVersion>
+      </Description>
+    </em:targetApplication>
+
+  </Description>
+</RDF>