--- a/toolkit/mozapps/extensions/AddonUpdateChecker.jsm
+++ b/toolkit/mozapps/extensions/AddonUpdateChecker.jsm
@@ -407,16 +407,18 @@ function UpdateParser(aId, aUpdateKey, a
 
   LOG("Requesting " + aUrl);
   try {
     this.request = Cc["@mozilla.org/xmlextras/xmlhttprequest;1"].
                    createInstance(Ci.nsIXMLHttpRequest);
     this.request.open("GET", this.url, true);
     this.request.channel.notificationCallbacks = new CertUtils.BadCertHandler(!requireBuiltIn);
     this.request.channel.loadFlags |= Ci.nsIRequest.LOAD_BYPASS_CACHE;
+    // Prevent the request from writing to cache.
+    this.request.channel.loadFlags |= Ci.nsIRequest.INHIBIT_CACHING;
     this.request.overrideMimeType("text/xml");
     var self = this;
     this.request.addEventListener("load", function loadEventListener(event) { self.onLoad() }, false);
     this.request.addEventListener("error", function errorEventListener(event) { self.onError() }, false);
     this.request.send(null);
   }
   catch (e) {
     ERROR("Failed to request update manifest", e);