--- a/toolkit/themes/shared/in-content/common.inc.css
+++ b/toolkit/themes/shared/in-content/common.inc.css
@@ -188,16 +188,18 @@ xul|menulist {
   border: 1px solid var(--in-content-box-border-color);
   -moz-border-top-colors: none !important;
   -moz-border-right-colors: none !important;
   -moz-border-bottom-colors: none !important;
   -moz-border-left-colors: none !important;
   border-radius: 2px;
   background-color: var(--in-content-page-background);
   margin: 4px 8px;
+  /* Ensure font-size isn't overridden by widget styling (e.g. in forms.css) */
+  font-size: 1em;
 }
 
 xul|button,
 html|button {
   /* use the same margin of other elements for the alignment */
   margin-left: 4px;
   margin-right: 4px;
 }