diff --git a/common/modules/prefs.jsm b/common/modules/prefs.jsm
index 8df88684..3f865b61 100644
--- a/common/modules/prefs.jsm
+++ b/common/modules/prefs.jsm
@@ -26,7 +26,8 @@ var Prefs = Module("prefs", XPCOM([Ci.nsIObserver, Ci.nsISupportsWeakReference])
         this._prefContexts = [];
 
         this.branch = services.pref[defaults ? "getDefaultBranch" : "getBranch"](branch || "");
-        this.branch instanceof Ci.nsIPrefBranch2;
+        if ("nsIPrefBranch2" in Ci)
+            this.branch instanceof Ci.nsIPrefBranch2;
 
         this.defaults = defaults ? this : this.constructor(branch, true);