--- a/browser/components/sessionstore/SessionStore.jsm
+++ b/browser/components/sessionstore/SessionStore.jsm
@@ -205,16 +205,27 @@ function debug(aMsg) {
  * A global value to tell that fingerprinting resistance is enabled or not.
  * If it's enabled, the session restore won't restore the window's size and
  * size mode.
  * This value is controlled by preference privacy.resistFingerprinting.
  */
 var gResistFingerprintingEnabled = false;
 
 this.SessionStore = {
+  /**
+   * Returns the counts of windows and tabs restorable from previous session
+   *
+   * @returns {Object.<string, number>} an object with the keys:
+   *            - windows: number of restorable windows
+   *            - tabs: number of restorable tabs
+   */
+  get restorableObjectsCount() {
+    return SessionStoreInternal.restorableObjectsCount;
+  },
+
   get promiseInitialized() {
     return SessionStoreInternal.promiseInitialized;
   },
 
   get promiseAllWindowsRestored() {
     return SessionStoreInternal.promiseAllWindowsRestored;
   },
 
@@ -547,16 +558,27 @@ var SessionStoreInternal = {
     deferred.promise = new Promise((resolve, reject) => {
       deferred.resolve = resolve;
       deferred.reject = reject;
     });
 
     return deferred;
   })(),
 
+  /**
+   * Returns the counts of windows and tabs restorable from previous session
+   *
+   * @returns {Object.<string, number>} an object with the keys:
+   *            - windows: number of restorable windows
+   *            - tabs: number of restorable tabs
+   */
+  get restorableObjectsCount() {
+    return LastSession.restorableObjectsCount;
+  },
+
   get promiseAllWindowsRestored() {
     return this._deferredAllWindowsRestored.promise;
   },
 
   // Promise that is resolved when we're ready to initialize
   // and restore the session.
   _promiseReadyForInitialization: null,
 
@@ -5084,16 +5106,29 @@ var DirtyWindows = {
 
 // The state from the previous session (after restoring pinned tabs). This
 // state is persisted and passed through to the next session during an app
 // restart to make the third party add-on warning not trash the deferred
 // session
 var LastSession = {
   _state: null,
 
+  /**
+   * Returns the counts of windows and tabs in the session
+   *
+   * @returns {Object.<string, number>} an object with the keys:
+   *            - windows: number of restorable windows
+   *            - tabs: number of restorable tabs
+   */
+  get restorableObjectsCount() {
+    let windows = this._state.windows.length;
+    let tabs = this._state.windows.reduce((acc, win) => acc + win.tabs.length, 0);
+    return {windows, tabs};
+  },
+
   get canRestore() {
     return !!this._state;
   },
 
   getState() {
     return this._state;
   },