diff --git a/extensions/b2g/viewer.html b/extensions/b2g/viewer.html
index 9f8c3782c..5a7867888 100644
--- a/extensions/b2g/viewer.html
+++ b/extensions/b2g/viewer.html
@@ -51,8 +51,8 @@ limitations under the License.
 
         <input type="number" id="pageNumber" class="toolbarField pageNumber" value="1" size="4" min="1">
 
-        <button class="toolbarButton zoomOut" title="Zoom Out" data-l10n-id="zoom_out"></button>
-        <button class="toolbarButton zoomIn" title="Zoom In" data-l10n-id="zoom_in"></button>
+        <button class="toolbarButton zoomOut" title="Zoom Out" id="zoom_out" data-l10n-id="zoom_out"></button>
+        <button class="toolbarButton zoomIn" title="Zoom In" id="zoom_in" data-l10n-id="zoom_in"></button>
       </footer>
     </section>