--- a/browser/components/sessionstore/SessionStore.jsm
+++ b/browser/components/sessionstore/SessionStore.jsm
@@ -821,16 +821,17 @@ let SessionStoreInternal = {
         this.saveStateDelayed(win);
         break;
       case "oop-browser-crashed":
         this.onBrowserCrashed(win, target);
         break;
       case "XULFrameLoaderCreated":
         if (target.tagName == "browser" && target.frameLoader && target.permanentKey) {
           this._lastKnownFrameLoader.set(target.permanentKey, target.frameLoader);
+          this.resetEpoch(target);
         }
         break;
       default:
         throw new Error(`unhandled event ${aEvent.type}?`);
     }
     this._clearRestoringWindows();
   },
 
@@ -3671,16 +3672,24 @@ let SessionStoreInternal = {
    * compare the epoch received with the message to the <browser> element's
    * epoch. This function does that, and returns true if |epoch| is up-to-date
    * with respect to |browser|.
    */
   isCurrentEpoch: function (browser, epoch) {
     return this.getCurrentEpoch(browser) == epoch;
   },
 
+  /**
+   * Resets the epoch for a given <browser>. We need to this every time we
+   * receive a hint that a new docShell has been loaded into the browser as
+   * the frame script starts out with epoch=0.
+   */
+  resetEpoch(browser) {
+    this._browserEpochs.delete(browser.permanentKey);
+  }
 };
 
 /**
  * Priority queue that keeps track of a list of tabs to restore and returns
  * the tab we should restore next, based on priority rules. We decide between
  * pinned, visible and hidden tabs in that and FIFO order. Hidden tabs are only
  * restored with restore_hidden_tabs=true.
  */