--- a/browser/base/content/browser-addons.js
+++ b/browser/base/content/browser-addons.js
@@ -409,38 +409,46 @@ var gXPInstallObserver = {
       let secHistogram = Services.telemetry.getHistogramById("SECURITY_UI");
       secHistogram.add(Ci.nsISecurityUITelemetry.WARNING_ADDON_ASKING_PREVENTED);
       let popup = PopupNotifications.show(browser, notificationID,
                                           messageString, anchorID,
                                           null, null, options);
       removeNotificationOnEnd(popup, installInfo.installs);
       break; }
     case "addon-install-blocked": {
-      messageString = gNavigatorBundle.getFormattedString("xpinstallPromptMessage.header", ["<>"]);
-      let unknown = gNavigatorBundle.getString("xpinstallPromptMessage.unknown");
-      options.name = options.displayURI && options.displayURI.displayHost || unknown;
+      let hasHost = !!options.displayURI;
+      if (hasHost) {
+        messageString = gNavigatorBundle.getFormattedString("xpinstallPromptMessage.header", ["<>"]);
+        options.name = options.displayURI.displayHost;
+      } else {
+        messageString = gNavigatorBundle.getString("xpinstallPromptMessage.header.unknown");
+      }
       // displayURI becomes it's own label, so we unset it for this panel. It will become part of the
       // messageString above.
       options.displayURI = undefined;
 
       options.eventCallback = (topic) => {
         if (topic !== "showing") {
           return;
         }
         let doc = browser.ownerDocument;
         let message = doc.getElementById("addon-install-blocked-message");
         // We must remove any prior use of this panel message in this window.
         while (message.firstChild) {
           message.firstChild.remove();
         }
-        let text = gNavigatorBundle.getString("xpinstallPromptMessage.message");
-        let b = doc.createElementNS("http://www.w3.org/1999/xhtml", "b");
-        b.textContent = options.name;
-        let fragment = BrowserUtils.getLocalizedFragment(doc, text, b);
-        message.appendChild(fragment);
+        if (hasHost) {
+          let text = gNavigatorBundle.getString("xpinstallPromptMessage.message");
+          let b = doc.createElementNS("http://www.w3.org/1999/xhtml", "b");
+          b.textContent = options.name;
+          let fragment = BrowserUtils.getLocalizedFragment(doc, text, b);
+          message.appendChild(fragment);
+        } else {
+          message.textContent = gNavigatorBundle.getString("xpinstallPromptMessage.message.unknown");
+        }
         let learnMore = doc.getElementById("addon-install-blocked-info");
         learnMore.textContent = gNavigatorBundle.getString("xpinstallPromptMessage.learnMore");
         learnMore.setAttribute("href", Services.urlFormatter.formatURLPref("app.support.baseURL") + "unlisted-extensions-risks");
       };
 
       let secHistogram = Services.telemetry.getHistogramById("SECURITY_UI");
       action = {
         label: gNavigatorBundle.getString("xpinstallPromptMessage.install"),

--- a/browser/locales/en-US/chrome/browser/browser.properties
+++ b/browser/locales/en-US/chrome/browser/browser.properties
@@ -20,20 +20,18 @@ contextMenuSearch.accesskey=S
 
 bookmarkAllTabsDefault=[Folder Name]
 
 xpinstallPromptMessage=%S prevented this site from asking you to install software on your computer.
 # LOCALIZATION NOTE (xpinstallPromptMessage.header)
 # The string contains the hostname of the site the add-on is being installed from.
 xpinstallPromptMessage.header=Allow %S to install an add-on?
 xpinstallPromptMessage.message=You are attempting to install an add-on from %S. Make sure you trust this site before continuing.
-# LOCALIZATION NOTE (xpinstallPromptMessage.unknown)
-# This string is used in xpinstallPromptMessage.header and xpinstallPromptMessage.message when the domain is not available,
-# for example in case of local installs with drag and drop.
-xpinstallPromptMessage.unknown=an unknown site
+xpinstallPromptMessage.header.unknown=Allow an unknown site to install an add-on?
+xpinstallPromptMessage.message.unknown=You are attempting to install an add-on from an unknown site. Make sure you trust this site before continuing.
 xpinstallPromptMessage.learnMore=Learn more about installing add-ons safely
 xpinstallPromptMessage.dontAllow=Don’t Allow
 xpinstallPromptMessage.dontAllow.accesskey=D
 # Accessibility Note:
 # Be sure you do not choose an accesskey that is used elsewhere in the active context (e.g. main menu bar, submenu of the warning popup button)
 # See https://website-archive.mozilla.org/www.mozilla.org/access/access/keyboard/ for details
 xpinstallPromptMessage.install=Continue to Installation
 xpinstallPromptMessage.install.accesskey=C