--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -4238,29 +4238,41 @@
 
             onSwapDocShells(ourBrowser, otherBrowser) {
               // This event fires before the swap. ourBrowser is from
               // our window. We save the state of otherBrowser since ourBrowser
               // needs to take on that state at the end of the swap.
 
               let otherTabbrowser = otherBrowser.ownerGlobal.gBrowser;
               let otherState;
+              let pendingTabChild = false;
               if (otherTabbrowser && otherTabbrowser._switcher) {
                 let otherTab = otherTabbrowser.getTabForBrowser(otherBrowser);
-                otherState = otherTabbrowser._switcher.getTabState(otherTab);
+                let otherSwitcher = otherTabbrowser._switcher;
+                otherState = otherSwitcher.getTabState(otherTab);
+                pendingTabChild = otherSwitcher.pendingTabChild.has(otherBrowser);
+
+                if (pendingTabChild) {
+                  this.assert(otherState == this.STATE_LOADING);
+                }
+
+                otherSwitcher.pendingTabChild.delete(otherBrowser);
               } else {
                 otherState = (otherBrowser.docShellIsActive
                               ? this.STATE_LOADED
                               : this.STATE_UNLOADED);
               }
 
               if (!this.swapMap) {
                 this.swapMap = new WeakMap();
               }
-              this.swapMap.set(otherBrowser, otherState);
+              this.swapMap.set(otherBrowser, {
+                state: otherState,
+                pendingTabChild,
+              });
             },
 
             onEndSwapDocShells(ourBrowser, otherBrowser) {
               // The swap has happened. We reset the loadingTab in
               // case it has been swapped. We also set ourBrowser's state
               // to whatever otherBrowser's state was before the swap.
 
               if (this.loadTimer) {
@@ -4269,22 +4281,27 @@
                 // ready yet. Typically it will already be ready
                 // though. If it's not, we're probably in a new window,
                 // in which case we have no other tabs to display anyway.
                 this.clearTimer(this.loadTimer);
                 this.loadTimer = null;
               }
               this.loadingTab = null;
 
-              let otherState = this.swapMap.get(otherBrowser);
+              let { state: otherState, pendingTabChild } =
+                this.swapMap.get(otherBrowser);
+
               this.swapMap.delete(otherBrowser);
 
               let ourTab = this.tabbrowser.getTabForBrowser(ourBrowser);
               if (ourTab) {
                 this.setTabStateNoAction(ourTab, otherState);
+                if (pendingTabChild) {
+                  this.pendingTabChild.add(ourTab.linkedBrowser);
+                }
               }
             },
 
             shouldActivateDocShell(browser) {
               let tab = this.tabbrowser.getTabForBrowser(browser);
               let state = this.getTabState(tab);
               return state == this.STATE_LOADING || state == this.STATE_LOADED;
             },