diff --git a/l10n/en-US/viewer.ftl b/l10n/en-US/viewer.ftl
index d0efeec892cbd..9242a298a2d3f 100644
--- a/l10n/en-US/viewer.ftl
+++ b/l10n/en-US/viewer.ftl
@@ -351,6 +351,8 @@ pdfjs-editor-stamp-add-image-button =
 pdfjs-editor-stamp-add-image-button-label = Add image
 # This refers to the thickness of the line used for free highlighting (not bound to text)
 pdfjs-editor-free-highlight-thickness-input = Thickness
+pdfjs-editor-free-highlight-thickness-title =
+    .title = Change thickness when highlighting items other than text
 
 pdfjs-free-text =
     .aria-label = Text Editor
diff --git a/web/viewer.html b/web/viewer.html
index 30a4cd7bcacc7..1b6451c91a218 100644
--- a/web/viewer.html
+++ b/web/viewer.html
@@ -179,7 +179,7 @@
             <div id="editorHighlightThickness">
               <label for="editorFreeHighlightThickness" class="editorParamsLabel" data-l10n-id="pdfjs-editor-free-highlight-thickness-input">Thickness</label>
               <div class="thicknessPicker">
-                <input type="range" id="editorFreeHighlightThickness" class="editorParamsSlider" value="12" min="8" max="24" step="1" tabindex="101">
+                <input type="range" id="editorFreeHighlightThickness" class="editorParamsSlider" data-l10n-id="pdfjs-editor-free-highlight-thickness-title" value="12" min="8" max="24" step="1" tabindex="101">
               </div>
             </div>
           </div>