diff --git a/scss/_forms.scss b/scss/_forms.scss
index 1f68383f92..9ea0ebc674 100644
--- a/scss/_forms.scss
+++ b/scss/_forms.scss
@@ -133,6 +133,7 @@ output {
   // Placeholder
   &::placeholder {
     color: $input-color-placeholder;
+    opacity: 1; // Override Firefox's unusual default opacity; see https://github.com/twbs/bootstrap/pull/11526
   }
 
   // Disabled and read-only inputs