fixup! MB 112: Updater customization for Mullvad Browser

Bug 271: Do not open update override page in a new tab, and show in
about:mullvad-browser instead.

browser/components/BrowserContentHandler.sys.mjs

@@ -666,6 +666,23 @@ nsBrowserContentHandler.prototype = {
       }
     }
 
+    // Retrieve the home page early so we can compare it against
+    // about:mullvad-browser to decide whether or not we need an override page
+    // (second tab) after an update was applied.
+    var startPage = "";
+    try {
+      var choice = prefb.getIntPref("browser.startup.page");
+      if (choice == 1 || choice == 3) {
+        startPage = lazy.HomePage.get();
+      }
+    } catch (e) {
+      console.error(e);
+    }
+
+    if (startPage == "about:blank") {
+      startPage = "";
+    }
+
     var override;
     var overridePage = "";
     var additionalPage = "";
@@ -734,6 +751,17 @@ nsBrowserContentHandler.prototype = {
               "%OLD_BASE_BROWSER_VERSION%",
               old_forkVersion
             );
+            if (overridePage && AppConstants.BASE_BROWSER_UPDATE) {
+              // Mullvad Browser, copied from tor-browser: Instead of opening
+              // the post-update "override page" directly, we include a link in
+              // about:mullvad-browser.
+              prefb.setCharPref("mullvadbrowser.post_update.url", overridePage);
+              prefb.setBoolPref("mullvadbrowser.post_update.shouldNotify", true);
+              // If the user's homepage is about:tor, we will inform them
+              // about the update on that page; otherwise, we arrange to
+              // open about:tor in a secondary tab.
+              overridePage = startPage === "about:mullvad-browser" ? "" : "about:mullvad-browser";
+            }
             break;
           case OVERRIDE_NEW_BUILD_ID:
             if (lazy.UpdateManager.readyUpdate) {
@@ -806,20 +834,6 @@ nsBrowserContentHandler.prototype = {
       }
     }
 
-    var startPage = "";
-    try {
-      var choice = prefb.getIntPref("browser.startup.page");
-      if (choice == 1 || choice == 3) {
-        startPage = lazy.HomePage.get();
-      }
-    } catch (e) {
-      console.error(e);
-    }
-
-    if (startPage == "about:blank") {
-      startPage = "";
-    }
-
     let skipStartPage =
       override == OVERRIDE_NEW_PROFILE &&
       prefb.getBoolPref("browser.startup.firstrunSkipsHomepage");