diff --git a/browser_patches/firefox-beta/BUILD_NUMBER b/browser_patches/firefox-beta/BUILD_NUMBER
index 1044a570b1..d66d2bcf90 100644
--- a/browser_patches/firefox-beta/BUILD_NUMBER
+++ b/browser_patches/firefox-beta/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1262
-Changed: max@schmitt.mx Tue Jun  8 21:07:04 UTC 2021
+1263
+Changed: max@schmitt.mx Thu Jun 10 14:40:52 UTC 2021
diff --git a/browser_patches/firefox-beta/juggler/content/Runtime.js b/browser_patches/firefox-beta/juggler/content/Runtime.js
index 5a37469510..ab5ddb6918 100644
--- a/browser_patches/firefox-beta/juggler/content/Runtime.js
+++ b/browser_patches/firefox-beta/juggler/content/Runtime.js
@@ -295,10 +295,12 @@ class ExecutionContext {
     this._id = generateId();
     this._auxData = auxData;
     this._jsonStringifyObject = this._debuggee.executeInGlobal(`((stringify, object) => {
-      const oldToJson = Date.prototype.toJSON;
+      const oldToJSON = Date.prototype.toJSON;
       Date.prototype.toJSON = undefined;
-      const oldArrayToJson = Array.prototype.toJSON;
-      Array.prototype.toJSON = undefined;
+      const oldArrayToJSON = Array.prototype.toJSON;
+      const oldArrayHadToJSON = Array.prototype.hasOwnProperty('toJSON');
+      if (oldArrayHadToJSON)
+        Array.prototype.toJSON = undefined;
 
       let hasSymbol = false;
       const result = stringify(object, (key, value) => {
@@ -307,8 +309,9 @@ class ExecutionContext {
         return value;
       });
 
-      Date.prototype.toJSON = oldToJson;
-      Array.prototype.toJSON = oldArrayToJson;
+      Date.prototype.toJSON = oldToJSON;
+      if (oldArrayHadToJSON)
+        Array.prototype.toJSON = oldArrayToJSON;
 
       return hasSymbol ? undefined : result;
     }).bind(null, JSON.stringify.bind(JSON))`).return;
diff --git a/browser_patches/firefox/BUILD_NUMBER b/browser_patches/firefox/BUILD_NUMBER
index cadea36ceb..56893356b3 100644
--- a/browser_patches/firefox/BUILD_NUMBER
+++ b/browser_patches/firefox/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1270
-Changed: max@schmitt.mx Tue Jun  8 20:11:01 UTC 2021
+1271
+Changed: max@schmitt.mx Thu Jun 10 14:39:06 UTC 2021
diff --git a/browser_patches/firefox/juggler/content/Runtime.js b/browser_patches/firefox/juggler/content/Runtime.js
index 5a37469510..ab5ddb6918 100644
--- a/browser_patches/firefox/juggler/content/Runtime.js
+++ b/browser_patches/firefox/juggler/content/Runtime.js
@@ -295,10 +295,12 @@ class ExecutionContext {
     this._id = generateId();
     this._auxData = auxData;
     this._jsonStringifyObject = this._debuggee.executeInGlobal(`((stringify, object) => {
-      const oldToJson = Date.prototype.toJSON;
+      const oldToJSON = Date.prototype.toJSON;
       Date.prototype.toJSON = undefined;
-      const oldArrayToJson = Array.prototype.toJSON;
-      Array.prototype.toJSON = undefined;
+      const oldArrayToJSON = Array.prototype.toJSON;
+      const oldArrayHadToJSON = Array.prototype.hasOwnProperty('toJSON');
+      if (oldArrayHadToJSON)
+        Array.prototype.toJSON = undefined;
 
       let hasSymbol = false;
       const result = stringify(object, (key, value) => {
@@ -307,8 +309,9 @@ class ExecutionContext {
         return value;
       });
 
-      Date.prototype.toJSON = oldToJson;
-      Array.prototype.toJSON = oldArrayToJson;
+      Date.prototype.toJSON = oldToJSON;
+      if (oldArrayHadToJSON)
+        Array.prototype.toJSON = oldArrayToJSON;
 
       return hasSymbol ? undefined : result;
     }).bind(null, JSON.stringify.bind(JSON))`).return;