diff --git a/toolkit/themes/windows/global/popup.css b/toolkit/themes/windows/global/popup.css
index 8049ffad3931..a27faa05ff14 100644
--- a/toolkit/themes/windows/global/popup.css
+++ b/toolkit/themes/windows/global/popup.css
@@ -103,11 +103,13 @@ panel[type="arrow"][side="right"] {
   -moz-box-orient: vertical;
 }
 
+@supports not -moz-bool-pref("browser.proton.enabled") {
 @media (-moz-os-version: windows-win7) {
 .panel-arrowcontent {
   border-radius: 4px;
 }
 }
+}
 
 panel[type="arrow"].panel-no-padding::part(arrowcontent) {
   padding: 0;