diff --git a/browser/components/sessionstore/src/SessionStore.jsm b/browser/components/sessionstore/src/SessionStore.jsm
index 54d6263e518..5a00b61a18f 100644
--- a/browser/components/sessionstore/src/SessionStore.jsm
+++ b/browser/components/sessionstore/src/SessionStore.jsm
@@ -311,9 +311,6 @@ let SessionStoreInternal = {
   // states for all recently closed windows
   _closedWindows: [],
 
-  // not-"dirty" windows usually don't need to have their data updated
-  _dirtyWindows: {},
-
   // collection of session states yet to be restored
   _statesToRestore: {},
 
@@ -1000,7 +997,7 @@ let SessionStoreInternal = {
     var activeWindow = this._getMostRecentBrowserWindow();
     if (activeWindow)
       this.activeWindowSSiCache = activeWindow.__SSi || "";
-    this._dirtyWindows = [];
+    DirtyWindows.clear();
   },
 
   /**
@@ -2544,14 +2541,14 @@ let SessionStoreInternal = {
       this._forEachBrowserWindow(function(aWindow) {
         if (!this._isWindowLoaded(aWindow)) // window data is still in _statesToRestore
           return;
-        if (aUpdateAll || this._dirtyWindows[aWindow.__SSi] || aWindow == activeWindow) {
+        if (aUpdateAll || DirtyWindows.has(aWindow) || aWindow == activeWindow) {
           this._collectWindowData(aWindow);
         }
         else { // always update the window features (whose change alone never triggers a save operation)
           this._updateWindowFeatures(aWindow);
         }
       });
-      this._dirtyWindows = [];
+      DirtyWindows.clear();
     }
 
     // collect the data for all windows
@@ -2682,7 +2679,7 @@ let SessionStoreInternal = {
       this._windows[aWindow.__SSi].__lastSessionWindowID =
         aWindow.__SS_lastSessionWindowID;
 
-    this._dirtyWindows[aWindow.__SSi] = false;
+    DirtyWindows.remove(aWindow);
   },
 
   /* ........ Restoring Functionality .............. */
@@ -3010,7 +3007,7 @@ let SessionStoreInternal = {
 
       // It's important to set the window state to dirty so that
       // we collect their data for the first time when saving state.
-      this._dirtyWindows[aWindow.__SSi] = true;
+      DirtyWindows.add(aWindow);
     }
 
     if (aTabs.length == 0) {
@@ -3701,7 +3698,7 @@ let SessionStoreInternal = {
    */
   saveStateDelayed: function ssi_saveStateDelayed(aWindow = null, aDelay = 2000) {
     if (aWindow) {
-      this._dirtyWindows[aWindow.__SSi] = true;
+      DirtyWindows.add(aWindow);
     }
 
     if (!this._saveTimer) {
@@ -4689,6 +4686,28 @@ let DyingWindowCache = {
   }
 };
 
+// A weak set of dirty windows. We use it to determine which windows we need to
+// recollect data for when _getCurrentState() is called.
+let DirtyWindows = {
+  _data: new WeakMap(),
+
+  has: function (window) {
+    return this._data.has(window);
+  },
+
+  add: function (window) {
+    return this._data.set(window, true);
+  },
+
+  remove: function (window) {
+    this._data.delete(window);
+  },
+
+  clear: function (window) {
+    this._data.clear();
+  }
+};
+
 // A map storing the number of tabs last closed per windoow. This only
 // stores the most recent tab-close operation, and is used to undo
 // batch tab-closing operations.