diff --git a/browser_patches/firefox-stable/BUILD_NUMBER b/browser_patches/firefox-stable/BUILD_NUMBER
index ebdca68403..2d31641cc4 100644
--- a/browser_patches/firefox-stable/BUILD_NUMBER
+++ b/browser_patches/firefox-stable/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1251
-Changed: lushnikov@chromium.org Wed 12 May 2021 04:13:18 PM PDT
+1252
+Changed: joel.einbinder@gmail.com Fri 14 May 2021 08:01:19 AM PDT
diff --git a/browser_patches/firefox-stable/juggler/components/juggler.js b/browser_patches/firefox-stable/juggler/components/juggler.js
index 7fd1ca3ad6..caa0987930 100644
--- a/browser_patches/firefox-stable/juggler/components/juggler.js
+++ b/browser_patches/firefox-stable/juggler/components/juggler.js
@@ -45,12 +45,6 @@ CommandLineHandler.prototype = {
 
     const loadFrameScript = () => {
       Services.mm.loadFrameScript(FRAME_SCRIPT, true /* aAllowDelayedLoad */);
-      if (Cc["@mozilla.org/gfx/info;1"].getService(Ci.nsIGfxInfo).isHeadless) {
-        const styleSheetService = Cc["@mozilla.org/content/style-sheet-service;1"].getService(Components.interfaces.nsIStyleSheetService);
-        const ioService = Cc["@mozilla.org/network/io-service;1"].getService(Components.interfaces.nsIIOService);
-        const uri = ioService.newURI('chrome://juggler/content/content/hidden-scrollbars.css', null, null);
-        styleSheetService.loadAndRegisterSheet(uri, styleSheetService.AGENT_SHEET);
-      }
     };
 
     // Force create hidden window here, otherwise its creation later closes the web socket!
diff --git a/browser_patches/firefox-stable/juggler/content/FrameTree.js b/browser_patches/firefox-stable/juggler/content/FrameTree.js
index 0b2cce1998..928ad67def 100644
--- a/browser_patches/firefox-stable/juggler/content/FrameTree.js
+++ b/browser_patches/firefox-stable/juggler/content/FrameTree.js
@@ -42,6 +42,8 @@ class FrameTree {
       Ci.nsISupportsWeakReference,
     ]);
 
+    this._addedScrollbarsStylesheetSymbol = Symbol('_addedScrollbarsStylesheetSymbol');
+
     this._wdm = Cc["@mozilla.org/dom/workers/workerdebuggermanager;1"].createInstance(Ci.nsIWorkerDebuggerManager);
     this._wdmListener = {
       QueryInterface: ChromeUtils.generateQI([Ci.nsIWorkerDebuggerManagerListener]),
@@ -103,12 +105,24 @@ class FrameTree {
   }
 
   _onDOMWindowCreated(window) {
+    if (!window[this._addedScrollbarsStylesheetSymbol] && this.scrollbarsHidden) {
+      const styleSheetService = Cc["@mozilla.org/content/style-sheet-service;1"].getService(Components.interfaces.nsIStyleSheetService);
+      const ioService = Cc["@mozilla.org/network/io-service;1"].getService(Components.interfaces.nsIIOService);
+      const uri = ioService.newURI('chrome://juggler/content/content/hidden-scrollbars.css', null, null);
+      const sheet = styleSheetService.preloadSheet(uri, styleSheetService.AGENT_SHEET);
+      window.windowUtils.addSheet(sheet, styleSheetService.AGENT_SHEET);
+      window[this._addedScrollbarsStylesheetSymbol] = true;
+    }
     const frame = this._docShellToFrame.get(window.docShell) || null;
     if (!frame)
       return;
     frame._onGlobalObjectCleared();
   }
 
+  setScrollbarsHidden(hidden) {
+    this.scrollbarsHidden = hidden;
+  }
+
   _onWorkerCreated(workerDebugger) {
     // Note: we do not interoperate with firefox devtools.
     if (workerDebugger.isInitialized)
diff --git a/browser_patches/firefox-stable/juggler/content/PageAgent.js b/browser_patches/firefox-stable/juggler/content/PageAgent.js
index a58f8309a3..427fc0fbc0 100644
--- a/browser_patches/firefox-stable/juggler/content/PageAgent.js
+++ b/browser_patches/firefox-stable/juggler/content/PageAgent.js
@@ -62,7 +62,6 @@ class PageAgent {
     const docShell = frameTree.mainFrame().docShell();
     this._docShell = docShell;
     this._initialDPPX = docShell.contentViewer.overrideDPPX;
-    this._customScrollbars = null;
     this._dragging = false;
 
     // Dispatch frameAttached events for all initial frames
diff --git a/browser_patches/firefox-stable/juggler/content/main.js b/browser_patches/firefox-stable/juggler/content/main.js
index ec9da55e35..4e7f427a10 100644
--- a/browser_patches/firefox-stable/juggler/content/main.js
+++ b/browser_patches/firefox-stable/juggler/content/main.js
@@ -62,6 +62,10 @@ const applySetting = {
     docShell.allowJavascript = !javaScriptDisabled;
   },
 
+  scrollbarsHidden: (hidden) => {
+    frameTree.setScrollbarsHidden(hidden);
+  },
+
   colorScheme: (colorScheme) => {
     frameTree.setColorScheme(colorScheme);
   },
diff --git a/browser_patches/firefox-stable/juggler/protocol/BrowserHandler.js b/browser_patches/firefox-stable/juggler/protocol/BrowserHandler.js
index e915dc6deb..a7499b7cee 100644
--- a/browser_patches/firefox-stable/juggler/protocol/BrowserHandler.js
+++ b/browser_patches/firefox-stable/juggler/protocol/BrowserHandler.js
@@ -233,6 +233,10 @@ class BrowserHandler {
     await this._targetRegistry.browserContextForId(browserContextId).setDefaultViewport(nullToUndefined(viewport));
   }
 
+  async ['Browser.setScrollbarsHidden']({browserContextId, hidden}) {
+    await this._targetRegistry.browserContextForId(browserContextId).applySetting('scrollbarsHidden', nullToUndefined(hidden));
+  }
+
   async ['Browser.addScriptToEvaluateOnNewDocument']({browserContextId, script}) {
     await this._targetRegistry.browserContextForId(browserContextId).addScriptToEvaluateOnNewDocument(script);
   }
diff --git a/browser_patches/firefox-stable/juggler/protocol/Protocol.js b/browser_patches/firefox-stable/juggler/protocol/Protocol.js
index 1dcd2f916b..8a70316198 100644
--- a/browser_patches/firefox-stable/juggler/protocol/Protocol.js
+++ b/browser_patches/firefox-stable/juggler/protocol/Protocol.js
@@ -364,6 +364,12 @@ const Browser = {
         viewport: t.Nullable(pageTypes.Viewport),
       }
     },
+    'setScrollbarsHidden': {
+      params: {
+        browserContextId: t.Optional(t.String),
+        hidden: t.Boolean,
+      }
+    },
     'addScriptToEvaluateOnNewDocument': {
       params: {
         browserContextId: t.Optional(t.String),
diff --git a/browser_patches/firefox/BUILD_NUMBER b/browser_patches/firefox/BUILD_NUMBER
index 83f8b0fc0a..2f2121d2c4 100644
--- a/browser_patches/firefox/BUILD_NUMBER
+++ b/browser_patches/firefox/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1261
-Changed: lushnikov@chromium.org Wed 12 May 2021 04:12:52 PM PDT
+1262
+Changed: joel.einbinder@gmail.com Fri 14 May 2021 08:00:09 AM PDT
diff --git a/browser_patches/firefox/juggler/components/juggler.js b/browser_patches/firefox/juggler/components/juggler.js
index 7fd1ca3ad6..caa0987930 100644
--- a/browser_patches/firefox/juggler/components/juggler.js
+++ b/browser_patches/firefox/juggler/components/juggler.js
@@ -45,12 +45,6 @@ CommandLineHandler.prototype = {
 
     const loadFrameScript = () => {
       Services.mm.loadFrameScript(FRAME_SCRIPT, true /* aAllowDelayedLoad */);
-      if (Cc["@mozilla.org/gfx/info;1"].getService(Ci.nsIGfxInfo).isHeadless) {
-        const styleSheetService = Cc["@mozilla.org/content/style-sheet-service;1"].getService(Components.interfaces.nsIStyleSheetService);
-        const ioService = Cc["@mozilla.org/network/io-service;1"].getService(Components.interfaces.nsIIOService);
-        const uri = ioService.newURI('chrome://juggler/content/content/hidden-scrollbars.css', null, null);
-        styleSheetService.loadAndRegisterSheet(uri, styleSheetService.AGENT_SHEET);
-      }
     };
 
     // Force create hidden window here, otherwise its creation later closes the web socket!
diff --git a/browser_patches/firefox/juggler/content/FrameTree.js b/browser_patches/firefox/juggler/content/FrameTree.js
index 0b2cce1998..928ad67def 100644
--- a/browser_patches/firefox/juggler/content/FrameTree.js
+++ b/browser_patches/firefox/juggler/content/FrameTree.js
@@ -42,6 +42,8 @@ class FrameTree {
       Ci.nsISupportsWeakReference,
     ]);
 
+    this._addedScrollbarsStylesheetSymbol = Symbol('_addedScrollbarsStylesheetSymbol');
+
     this._wdm = Cc["@mozilla.org/dom/workers/workerdebuggermanager;1"].createInstance(Ci.nsIWorkerDebuggerManager);
     this._wdmListener = {
       QueryInterface: ChromeUtils.generateQI([Ci.nsIWorkerDebuggerManagerListener]),
@@ -103,12 +105,24 @@ class FrameTree {
   }
 
   _onDOMWindowCreated(window) {
+    if (!window[this._addedScrollbarsStylesheetSymbol] && this.scrollbarsHidden) {
+      const styleSheetService = Cc["@mozilla.org/content/style-sheet-service;1"].getService(Components.interfaces.nsIStyleSheetService);
+      const ioService = Cc["@mozilla.org/network/io-service;1"].getService(Components.interfaces.nsIIOService);
+      const uri = ioService.newURI('chrome://juggler/content/content/hidden-scrollbars.css', null, null);
+      const sheet = styleSheetService.preloadSheet(uri, styleSheetService.AGENT_SHEET);
+      window.windowUtils.addSheet(sheet, styleSheetService.AGENT_SHEET);
+      window[this._addedScrollbarsStylesheetSymbol] = true;
+    }
     const frame = this._docShellToFrame.get(window.docShell) || null;
     if (!frame)
       return;
     frame._onGlobalObjectCleared();
   }
 
+  setScrollbarsHidden(hidden) {
+    this.scrollbarsHidden = hidden;
+  }
+
   _onWorkerCreated(workerDebugger) {
     // Note: we do not interoperate with firefox devtools.
     if (workerDebugger.isInitialized)
diff --git a/browser_patches/firefox/juggler/content/PageAgent.js b/browser_patches/firefox/juggler/content/PageAgent.js
index a58f8309a3..427fc0fbc0 100644
--- a/browser_patches/firefox/juggler/content/PageAgent.js
+++ b/browser_patches/firefox/juggler/content/PageAgent.js
@@ -62,7 +62,6 @@ class PageAgent {
     const docShell = frameTree.mainFrame().docShell();
     this._docShell = docShell;
     this._initialDPPX = docShell.contentViewer.overrideDPPX;
-    this._customScrollbars = null;
     this._dragging = false;
 
     // Dispatch frameAttached events for all initial frames
diff --git a/browser_patches/firefox/juggler/content/main.js b/browser_patches/firefox/juggler/content/main.js
index ec9da55e35..4e7f427a10 100644
--- a/browser_patches/firefox/juggler/content/main.js
+++ b/browser_patches/firefox/juggler/content/main.js
@@ -62,6 +62,10 @@ const applySetting = {
     docShell.allowJavascript = !javaScriptDisabled;
   },
 
+  scrollbarsHidden: (hidden) => {
+    frameTree.setScrollbarsHidden(hidden);
+  },
+
   colorScheme: (colorScheme) => {
     frameTree.setColorScheme(colorScheme);
   },
diff --git a/browser_patches/firefox/juggler/protocol/BrowserHandler.js b/browser_patches/firefox/juggler/protocol/BrowserHandler.js
index e915dc6deb..a7499b7cee 100644
--- a/browser_patches/firefox/juggler/protocol/BrowserHandler.js
+++ b/browser_patches/firefox/juggler/protocol/BrowserHandler.js
@@ -233,6 +233,10 @@ class BrowserHandler {
     await this._targetRegistry.browserContextForId(browserContextId).setDefaultViewport(nullToUndefined(viewport));
   }
 
+  async ['Browser.setScrollbarsHidden']({browserContextId, hidden}) {
+    await this._targetRegistry.browserContextForId(browserContextId).applySetting('scrollbarsHidden', nullToUndefined(hidden));
+  }
+
   async ['Browser.addScriptToEvaluateOnNewDocument']({browserContextId, script}) {
     await this._targetRegistry.browserContextForId(browserContextId).addScriptToEvaluateOnNewDocument(script);
   }
diff --git a/browser_patches/firefox/juggler/protocol/Protocol.js b/browser_patches/firefox/juggler/protocol/Protocol.js
index 1dcd2f916b..8a70316198 100644
--- a/browser_patches/firefox/juggler/protocol/Protocol.js
+++ b/browser_patches/firefox/juggler/protocol/Protocol.js
@@ -364,6 +364,12 @@ const Browser = {
         viewport: t.Nullable(pageTypes.Viewport),
       }
     },
+    'setScrollbarsHidden': {
+      params: {
+        browserContextId: t.Optional(t.String),
+        hidden: t.Boolean,
+      }
+    },
     'addScriptToEvaluateOnNewDocument': {
       params: {
         browserContextId: t.Optional(t.String),