diff --git a/browser_patches/firefox-beta/BUILD_NUMBER b/browser_patches/firefox-beta/BUILD_NUMBER
index 946d188523..a64f43ae8a 100644
--- a/browser_patches/firefox-beta/BUILD_NUMBER
+++ b/browser_patches/firefox-beta/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1282
-Changed: joel.einbinder@gmail.com Wed 25 Aug 2021 08:16:37 AM PDT
+1283
+Changed: joel.einbinder@gmail.com Wed 25 Aug 2021 08:42:49 AM PDT
diff --git a/browser_patches/firefox-beta/juggler/TargetRegistry.js b/browser_patches/firefox-beta/juggler/TargetRegistry.js
index 84228109f4..d8f5dbbc89 100644
--- a/browser_patches/firefox-beta/juggler/TargetRegistry.js
+++ b/browser_patches/firefox-beta/juggler/TargetRegistry.js
@@ -169,6 +169,7 @@ class TargetRegistry {
         throw new Error(`Internal error: cannot find context for userContextId=${userContextId}`);
       const target = new PageTarget(this, window, tab, browserContext, openerTarget);
       target.updateUserAgent();
+      target.updatePlatform();
       target.updateJavaScriptDisabled();
       target.updateTouchOverride();
       target.updateColorSchemeOverride();
@@ -406,6 +407,10 @@ class PageTarget {
     this._linkedBrowser.browsingContext.customUserAgent = this._browserContext.defaultUserAgent;
   }
 
+  updatePlatform() {
+    this._linkedBrowser.browsingContext.customPlatform = this._browserContext.defaultPlatform;
+  }
+
   updateJavaScriptDisabled() {
     this._linkedBrowser.browsingContext.allowJavascript = !this._browserContext.javaScriptDisabled;
   }
@@ -695,6 +700,7 @@ class BrowserContext {
     this.defaultViewportSize = undefined;
     this.deviceScaleFactor = undefined;
     this.defaultUserAgent = null;
+    this.defaultPlatform = null;
     this.javaScriptDisabled = false;
     this.touchOverride = false;
     this.colorScheme = 'none';
@@ -773,6 +779,12 @@ class BrowserContext {
       page.updateUserAgent();
   }
 
+  setDefaultPlatform(platform) {
+    this.defaultPlatform = platform;
+    for (const page of this.pages)
+      page.updatePlatform();
+  }
+
   setJavaScriptDisabled(javaScriptDisabled) {
     this.javaScriptDisabled = javaScriptDisabled;
     for (const page of this.pages)
diff --git a/browser_patches/firefox-beta/juggler/protocol/BrowserHandler.js b/browser_patches/firefox-beta/juggler/protocol/BrowserHandler.js
index accad571a7..cbc1953fbe 100644
--- a/browser_patches/firefox-beta/juggler/protocol/BrowserHandler.js
+++ b/browser_patches/firefox-beta/juggler/protocol/BrowserHandler.js
@@ -221,6 +221,10 @@ class BrowserHandler {
     await this._targetRegistry.browserContextForId(browserContextId).setDefaultUserAgent(userAgent);
   }
 
+  async ['Browser.setPlatformOverride']({browserContextId, platform}) {
+    await this._targetRegistry.browserContextForId(browserContextId).setDefaultPlatform(platform);
+  }
+
   async ['Browser.setBypassCSP']({browserContextId, bypassCSP}) {
     await this._targetRegistry.browserContextForId(browserContextId).applySetting('bypassCSP', nullToUndefined(bypassCSP));
   }
diff --git a/browser_patches/firefox-beta/juggler/protocol/Protocol.js b/browser_patches/firefox-beta/juggler/protocol/Protocol.js
index 21421574f7..1efc2c8f0a 100644
--- a/browser_patches/firefox-beta/juggler/protocol/Protocol.js
+++ b/browser_patches/firefox-beta/juggler/protocol/Protocol.js
@@ -327,6 +327,12 @@ const Browser = {
         userAgent: t.Nullable(t.String),
       }
     },
+    'setPlatformOverride': {
+      params: {
+        browserContextId: t.Optional(t.String),
+        platform: t.Nullable(t.String),
+      }
+    },
     'setBypassCSP': {
       params: {
         browserContextId: t.Optional(t.String),
diff --git a/browser_patches/firefox/BUILD_NUMBER b/browser_patches/firefox/BUILD_NUMBER
index 026b2f0172..c7950901a1 100644
--- a/browser_patches/firefox/BUILD_NUMBER
+++ b/browser_patches/firefox/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1287
-Changed: joel.einbinder@gmail.com Wed 25 Aug 2021 08:15:20 AM PDT
+1288
+Changed: joel.einbinder@gmail.com Wed 25 Aug 2021 08:41:36 AM PDT
diff --git a/browser_patches/firefox/juggler/TargetRegistry.js b/browser_patches/firefox/juggler/TargetRegistry.js
index 84228109f4..d8f5dbbc89 100644
--- a/browser_patches/firefox/juggler/TargetRegistry.js
+++ b/browser_patches/firefox/juggler/TargetRegistry.js
@@ -169,6 +169,7 @@ class TargetRegistry {
         throw new Error(`Internal error: cannot find context for userContextId=${userContextId}`);
       const target = new PageTarget(this, window, tab, browserContext, openerTarget);
       target.updateUserAgent();
+      target.updatePlatform();
       target.updateJavaScriptDisabled();
       target.updateTouchOverride();
       target.updateColorSchemeOverride();
@@ -406,6 +407,10 @@ class PageTarget {
     this._linkedBrowser.browsingContext.customUserAgent = this._browserContext.defaultUserAgent;
   }
 
+  updatePlatform() {
+    this._linkedBrowser.browsingContext.customPlatform = this._browserContext.defaultPlatform;
+  }
+
   updateJavaScriptDisabled() {
     this._linkedBrowser.browsingContext.allowJavascript = !this._browserContext.javaScriptDisabled;
   }
@@ -695,6 +700,7 @@ class BrowserContext {
     this.defaultViewportSize = undefined;
     this.deviceScaleFactor = undefined;
     this.defaultUserAgent = null;
+    this.defaultPlatform = null;
     this.javaScriptDisabled = false;
     this.touchOverride = false;
     this.colorScheme = 'none';
@@ -773,6 +779,12 @@ class BrowserContext {
       page.updateUserAgent();
   }
 
+  setDefaultPlatform(platform) {
+    this.defaultPlatform = platform;
+    for (const page of this.pages)
+      page.updatePlatform();
+  }
+
   setJavaScriptDisabled(javaScriptDisabled) {
     this.javaScriptDisabled = javaScriptDisabled;
     for (const page of this.pages)
diff --git a/browser_patches/firefox/juggler/protocol/BrowserHandler.js b/browser_patches/firefox/juggler/protocol/BrowserHandler.js
index accad571a7..cbc1953fbe 100644
--- a/browser_patches/firefox/juggler/protocol/BrowserHandler.js
+++ b/browser_patches/firefox/juggler/protocol/BrowserHandler.js
@@ -221,6 +221,10 @@ class BrowserHandler {
     await this._targetRegistry.browserContextForId(browserContextId).setDefaultUserAgent(userAgent);
   }
 
+  async ['Browser.setPlatformOverride']({browserContextId, platform}) {
+    await this._targetRegistry.browserContextForId(browserContextId).setDefaultPlatform(platform);
+  }
+
   async ['Browser.setBypassCSP']({browserContextId, bypassCSP}) {
     await this._targetRegistry.browserContextForId(browserContextId).applySetting('bypassCSP', nullToUndefined(bypassCSP));
   }
diff --git a/browser_patches/firefox/juggler/protocol/Protocol.js b/browser_patches/firefox/juggler/protocol/Protocol.js
index 21421574f7..1efc2c8f0a 100644
--- a/browser_patches/firefox/juggler/protocol/Protocol.js
+++ b/browser_patches/firefox/juggler/protocol/Protocol.js
@@ -327,6 +327,12 @@ const Browser = {
         userAgent: t.Nullable(t.String),
       }
     },
+    'setPlatformOverride': {
+      params: {
+        browserContextId: t.Optional(t.String),
+        platform: t.Nullable(t.String),
+      }
+    },
     'setBypassCSP': {
       params: {
         browserContextId: t.Optional(t.String),