diff --git a/toolkit/content/widgets/arrowscrollbox.js b/toolkit/content/widgets/arrowscrollbox.js
index f9191af09f1b7a1654aff62807e7dad573afc172..f94a8b3dc5871fba5d0dbed5d487d6e32a1ff29a 100644
--- a/toolkit/content/widgets/arrowscrollbox.js
+++ b/toolkit/content/widgets/arrowscrollbox.js
@@ -98,6 +98,7 @@
 
       let slot = this.shadowRoot.querySelector("slot");
       let overflowObserver = new ResizeObserver(_ => {
+        if (this.id == 'tabbrowser-arrowscrollbox') return; // zen: do NOT underflow/overflow on tabbrowser-arrowscrollbox
         let contentSize =
           slot.getBoundingClientRect()[this.#verticalMode ? "height" : "width"];
         // NOTE(emilio): This should be contentSize > scrollClientSize, but due
@@ -639,7 +640,7 @@
 
     on_wheel(event) {
       // Don't consume the event if we can't scroll.
-      if (!this.overflowing) {
+      if (!this.overflowing || this.id === 'tabbrowser-arrowscrollbox') { // we handle this on ZenStartup
         return;
       }
 
