diff --git a/browser/themes/shared/tabbrowser/ctrlTab.css b/browser/themes/shared/tabbrowser/ctrlTab.css
index d016bb363f306d7f6b08554f5029d425c01ca358..0f8e4d0a1aa2f9958124ea75423f83eb6ef35afe 100644
--- a/browser/themes/shared/tabbrowser/ctrlTab.css
+++ b/browser/themes/shared/tabbrowser/ctrlTab.css
@@ -11,6 +11,7 @@
   --panel-border-color: transparent;
   --panel-padding: 20px 10px 10px;
   -moz-window-shadow: none;
+  --panel-shadow-margin: 0;
   background-color: transparent;
   @media not (-moz-platform: macos) {
     font-weight: var(--font-weight-bold);
