Bug 1544489 use separate strings to localize install prompt for unknown sites. r=flod,aswan

Differential Revision: https://phabricator.services.mozilla.com/D27606

--HG--
extra : moz-landing-system : lando

browser/base/content/browser-addons.js

@@ -414,9 +414,13 @@ var gXPInstallObserver = {
       removeNotificationOnEnd(popup, installInfo.installs);
       break; }
     case "addon-install-blocked": {
+      let hasHost = !!options.displayURI;
+      if (hasHost) {
         messageString = gNavigatorBundle.getFormattedString("xpinstallPromptMessage.header", ["<>"]);
-      let unknown = gNavigatorBundle.getString("xpinstallPromptMessage.unknown");
-      options.name = options.displayURI && options.displayURI.displayHost || unknown;
+        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;
@@ -431,11 +435,15 @@ var gXPInstallObserver = {
         while (message.firstChild) {
           message.firstChild.remove();
         }
+        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");

browser/locales/en-US/chrome/browser/browser.properties

@@ -25,10 +25,8 @@ xpinstallPromptMessage=%S prevented this site from asking you to install softwar
 # 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