--- a/widget/cocoa/nsNativeThemeCocoa.mm
+++ b/widget/cocoa/nsNativeThemeCocoa.mm
@@ -4094,16 +4094,22 @@ nsNativeThemeCocoa::GetMinimumWidgetSize
     case NS_THEME_PROGRESSBAR:
     {
       SInt32 barHeight = 0;
       ::GetThemeMetric(kThemeMetricNormalProgressBarThickness, &barHeight);
       aResult->SizeTo(0, barHeight);
       break;
     }
 
+    case NS_THEME_SEPARATOR:
+    {
+      aResult->SizeTo(1, 1);
+      break;
+    }
+
     case NS_THEME_TREETWISTY:
     case NS_THEME_TREETWISTYOPEN:
     {
       SInt32 twistyHeight = 0, twistyWidth = 0;
       ::GetThemeMetric(kThemeMetricDisclosureButtonWidth, &twistyWidth);
       ::GetThemeMetric(kThemeMetricDisclosureButtonHeight, &twistyHeight);
       aResult->SizeTo(twistyWidth, twistyHeight);
       *aIsOverridable = false;