Revert "Start WebExtension immediately"

This reverts commit 81a567a.

addon/bootstrap.js

@@ -43,8 +43,28 @@ const prefObserver = {
   }
 };
 
+const appStartupObserver = {
+  register() {
+    Services.obs.addObserver(this, "sessionstore-windows-restored", false); // eslint-disable-line mozilla/no-useless-parameters
+  },
+
+  unregister() {
+    Services.obs.removeObserver(this, "sessionstore-windows-restored", false); // eslint-disable-line mozilla/no-useless-parameters
+  },
+
+  observe() {
+    appStartupDone();
+    this.unregister();
+  }
+}
+
+const APP_STARTUP = 1;
 function startup(data, reason) { // eslint-disable-line no-unused-vars
-  appStartupDone();
+  if (reason === APP_STARTUP) {
+    appStartupObserver.register();
+  } else {
+    appStartupDone();
+  }
   prefObserver.register();
   addonResourceURI = data.resourceURI;
   // eslint-disable-next-line promise/catch-or-return