--- a/toolkit/mozapps/downloads/DownloadLastDir.jsm
+++ b/toolkit/mozapps/downloads/DownloadLastDir.jsm
@@ -78,24 +78,22 @@ let observer = {
   },
   observe: function (aSubject, aTopic, aData) {
     switch (aTopic) {
       case "private-browsing":
         if (aData == "enter")
           gDownloadLastDirFile = readLastDirPref();
         else if (aData == "exit") {
           gDownloadLastDirFile = null;
-          gDownloadLastDirStore = new Dict();
         }
         break;
       case "browser:purge-session-history":
         gDownloadLastDirFile = null;
         if (Services.prefs.prefHasUserValue(LAST_DIR_PREF))
           Services.prefs.clearUserPref(LAST_DIR_PREF);
-        gDownloadLastDirStore = new Dict();
         Services.contentPrefs.removePrefsByName(LAST_DIR_PREF);
         break;
     }
   }
 };
 
 let os = Components.classes["@mozilla.org/observer-service;1"]
                    .getService(Components.interfaces.nsIObserverService);
@@ -107,31 +105,23 @@ function readLastDirPref() {
     return Services.prefs.getComplexValue(LAST_DIR_PREF, nsILocalFile);
   }
   catch (e) {
     return null;
   }
 }
 
 let gDownloadLastDirFile = readLastDirPref();
-let gDownloadLastDirStore = new Dict();
 let gDownloadLastDir = {
   // compat shims
   get file() { return this.getFile(); },
   set file(val) { this.setFile(null, val); },
   getFile: function (aURI) {
     if (aURI) {
-      let lastDir;
-      if (pbSvc && pbSvc.privateBrowsingEnabled) {
-        let group = Services.contentPrefs.grouper.group(aURI);
-        lastDir = gDownloadLastDirStore.get(group, null);
-      }
-      if (!lastDir) {
-        lastDir = Services.contentPrefs.getPref(aURI, LAST_DIR_PREF);
-      }
+      let lastDir = Services.contentPrefs.getPref(aURI, LAST_DIR_PREF);
       if (lastDir) {
         var lastDirFile = Components.classes["@mozilla.org/file/local;1"]
                                     .createInstance(Components.interfaces.nsILocalFile);
         lastDirFile.initWithPath(lastDir);
         return lastDirFile;
       }
     }
     if (gDownloadLastDirFile && !gDownloadLastDirFile.exists())
@@ -139,22 +129,17 @@ let gDownloadLastDir = {
 
     if (pbSvc && pbSvc.privateBrowsingEnabled)
       return gDownloadLastDirFile;
     else
       return readLastDirPref();
   },
   setFile: function (aURI, aFile) {
     if (aURI) {
-      if (pbSvc && pbSvc.privateBrowsingEnabled) {
-        let group = Services.contentPrefs.grouper.group(aURI);
-        gDownloadLastDirStore.set(group, aFile.path);
-      } else {
-        Services.contentPrefs.setPref(aURI, LAST_DIR_PREF, aFile.path);
-      }
+      Services.contentPrefs.setPref(aURI, LAST_DIR_PREF, aFile.path);
     }
     if (pbSvc && pbSvc.privateBrowsingEnabled) {
       if (aFile instanceof Components.interfaces.nsIFile)
         gDownloadLastDirFile = aFile.clone();
       else
         gDownloadLastDirFile = null;
     } else {
       if (aFile instanceof Components.interfaces.nsIFile)