diff --git a/browser/themes/shared/customizableui/panelUIOverlay.inc.css b/browser/themes/shared/customizableui/panelUIOverlay.inc.css
index 8421302cc9d5..a53c9659b7cb 100644
--- a/browser/themes/shared/customizableui/panelUIOverlay.inc.css
+++ b/browser/themes/shared/customizableui/panelUIOverlay.inc.css
@@ -231,7 +231,7 @@ panelview:not([mainview]) .toolbarbutton-text,
 
 #PanelUI-contents {
   display: block;
-  flex: auto;
+  flex: 1 0 auto;
   margin-left: auto;
   margin-right: auto;
   padding: .5em 0;