diff --git a/extension/content/inject.js b/extension/content/inject.js
index 3e6665f..8aa9379 100644
--- a/extension/content/inject.js
+++ b/extension/content/inject.js
@@ -3137,14 +3137,14 @@ function disableDirectFullscreenPopover(videoController) {
   wrapper.removeAttribute("popover");
 }
 
-function enableDirectFullscreenPopover(videoController) {
+function enableFullscreenPopover(videoController, preferredMount) {
   if (!videoController || !videoController.video || !videoController.div) {
     return false;
   }
   var wrapper = videoController.div;
   if (typeof wrapper.showPopover !== "function") return false;
 
-  var normalMount = videoController.normalControllerMount;
+  var normalMount = preferredMount || videoController.normalControllerMount;
   var normalMountIsConnected = Boolean(
     normalMount &&
       (normalMount.isConnected ||
@@ -3185,12 +3185,6 @@ function syncControllerFullscreenMount(videoController) {
   var fullscreenElement = getFullscreenElement(doc);
   var targetMount = videoController.normalControllerMount;
 
-  if (fullscreenElement === video) {
-    if (enableDirectFullscreenPopover(videoController)) return true;
-  } else {
-    disableDirectFullscreenPopover(videoController);
-  }
-
   if (
     fullscreenElement &&
     (fullscreenElement === video ||
@@ -3203,6 +3197,18 @@ function syncControllerFullscreenMount(videoController) {
   }
 
   if (!targetMount) return false;
+
+  if (fullscreenElement) {
+    // Fullscreen elements and popovers both participate in the browser's top
+    // layer. Showing Speeder's host after the player enters fullscreen keeps it
+    // above provider-owned surfaces even when the provider clips descendants or
+    // creates a new fullscreen stacking context (notably Firefox + YouTube).
+    // Browsers without the Popover API retain the player-local remount fallback.
+    if (enableFullscreenPopover(videoController, targetMount)) return true;
+  } else {
+    disableDirectFullscreenPopover(videoController);
+  }
+
   return remountControllerHost(videoController, targetMount);
 }
 
diff --git a/tests/inject-lifecycle.regression.test.js b/tests/inject-lifecycle.regression.test.js
index d099ae0..4c31e3c 100644
--- a/tests/inject-lifecycle.regression.test.js
+++ b/tests/inject-lifecycle.regression.test.js
@@ -443,6 +443,52 @@ describe("inject.js media/controller lifecycle regressions", () => {
     controller.controllerHostCleanup();
   });
 
+  it("promotes an ancestor-fullscreen controller into the browser top layer", async () => {
+    bootInject();
+    await settleLifecycle();
+
+    const fullscreenPlayer = document.createElement("div");
+    const video = document.createElement("video");
+    const wrapper = document.createElement("div");
+    const rect = makeRect(0, 0, 1280, 720);
+
+    fullscreenPlayer.append(video, wrapper);
+    document.body.appendChild(fullscreenPlayer);
+    [fullscreenPlayer, video].forEach((element) => {
+      setRect(element, rect);
+      setBoxMetrics(element, rect.width, rect.height);
+    });
+    wrapper.showPopover = vi.fn();
+    wrapper.hidePopover = vi.fn();
+
+    const controller = {
+      video,
+      div: wrapper,
+      normalControllerMount: fullscreenPlayer
+    };
+    window.setupControllerHostTracking(controller, wrapper, fullscreenPlayer);
+    Object.defineProperty(document, "fullscreenElement", {
+      configurable: true,
+      value: fullscreenPlayer
+    });
+
+    expect(window.syncControllerFullscreenMount(controller)).toBe(true);
+    expect(wrapper.parentNode).toBe(fullscreenPlayer);
+    expect(wrapper.showPopover).toHaveBeenCalledOnce();
+    expect(wrapper.getAttribute("popover")).toBe("manual");
+    expect(wrapper.classList.contains("vsc-fullscreen-popover")).toBe(true);
+
+    Object.defineProperty(document, "fullscreenElement", {
+      configurable: true,
+      value: null
+    });
+    window.syncControllerFullscreenMount(controller);
+    expect(wrapper.hidePopover).toHaveBeenCalledOnce();
+
+    wrapper.remove();
+    controller.controllerHostCleanup();
+  });
+
   it("preserves direct-video requestFullscreen semantics and overlays with a popover", async () => {
     bootInject();
     await settleLifecycle();