fixed font family in the popup (Firefox)

Former-commit-id: 12aaaf2aabf9c7046924b14b50a9ff581fbdcaa0

extension/ui/pages/pendings.css

@@ -6,7 +6,8 @@ body {
     margin: 0;
 }
 
-main {
+main,
+header {
     font-family: sans-serif;
 }