diff --git a/application/palemoon/base/content/tabbrowser.xml b/application/palemoon/base/content/tabbrowser.xml
index 6bc5a33ce9..52e39d898b 100644
--- a/application/palemoon/base/content/tabbrowser.xml
+++ b/application/palemoon/base/content/tabbrowser.xml
@@ -1972,7 +1972,9 @@
             setTimeout(function (tab, tabbrowser) {
               if (tab.parentNode &&
                   window.getComputedStyle(tab).maxWidth == "0.1px") {
-                NS_ASSERT(false, "Giving up waiting for the tab closing animation to finish (bug 608589)");
+                if (this.visible) {
+                  NS_ASSERT(false, "Giving up waiting for the tab closing animation to finish (bug 608589)");
+                }
                 tabbrowser._endRemoveTab(tab);
               }
             }, 3000, aTab, this);