diff --git a/web/viewer.js b/web/viewer.js
index 9b7d4749..c6a9766d 100644
--- a/web/viewer.js
+++ b/web/viewer.js
@@ -2295,6 +2295,7 @@ window.addEventListener('keydown', function keydown(evt) {
         PDFView.zoomIn();
         handled = true;
         break;
+      case 173: // FF/Mac '-'
       case 109: // FF '-'
       case 189: // Chrome '-'
         PDFView.zoomOut();