--- a/mobile/android/chrome/content/browser.js
+++ b/mobile/android/chrome/content/browser.js
@@ -5155,18 +5155,18 @@ var XPInstallObserver = {
     Services.obs.addObserver(this, "xpi-signature-changed", false);
     Services.obs.addObserver(this, "browser-delayed-startup-finished", false);
 
     AddonManager.addInstallListener(this);
   },
 
   observe: function(aSubject, aTopic, aData) {
     let installInfo, tab, host;
-    if (aSubject && aSubject instanceof Ci.amIWebInstallInfo) {
-      installInfo = aSubject;
+    if (aSubject && aSubject.wrappedJSObject) {
+      installInfo = aSubject.wrappedJSObject;
       tab = BrowserApp.getTabForBrowser(installInfo.browser);
       if (installInfo.originatingURI) {
         host = installInfo.originatingURI.host;
       }
     }
 
     let strings = Strings.browser;
     let brandShortName = Strings.brand.GetStringFromName("brandShortName");