diff --git a/ext/fg/js/frame-offset-forwarder.js b/ext/fg/js/frame-offset-forwarder.js
index 1a2f3e1e..9b68d34e 100644
--- a/ext/fg/js/frame-offset-forwarder.js
+++ b/ext/fg/js/frame-offset-forwarder.js
@@ -105,7 +105,10 @@ class FrameOffsetForwarder {
                     return element;
                 }
 
-                const shadowRoot = element.shadowRoot;
+                const shadowRoot = (
+                    element.shadowRoot ||
+                    element.openOrClosedShadowRoot // Available to Firefox 63+ for WebExtensions
+                );
                 if (shadowRoot) {
                     for (const child of shadowRoot.children) {
                         if (child.nodeType === ELEMENT_NODE) {
diff --git a/ext/fg/js/popup.js b/ext/fg/js/popup.js
index d45c7c2a..9e9debd8 100644
--- a/ext/fg/js/popup.js
+++ b/ext/fg/js/popup.js
@@ -530,7 +530,11 @@ class Popup {
     _getFrameParentElement() {
         const defaultParent = document.body;
         const fullscreenElement = DOM.getFullscreenElement();
-        if (fullscreenElement === null || fullscreenElement.shadowRoot) {
+        if (
+            fullscreenElement === null ||
+            fullscreenElement.shadowRoot ||
+            fullscreenElement.openOrClosedShadowRoot // Available to Firefox 63+ for WebExtensions
+        ) {
             return defaultParent;
         }