[BUG] Firefox: strange undefined toJSON property on JS objects

[creage]

Context:

• Playwright Version: 1.12.1
• Operating System: Windows
• Node.js version: 14.6.1
• Browser: Firefox

Describe the bug
After evaluating some custom code in the context of the page, some of JS objects receive strange toJSON property, which is undefined. Started to happen after upgrade to 1.12. And this happens on Firefox ONLY.

[mxschmitt]

Caused by #6767

[creage]

Oh gosh, this reminds me old issues you guys might not even heard about 😄

That library taught all of us one thing - you should never modify objects you don't own.

[mxschmitt]

This was the original reason why we implemented this fix (prototype.js) to bypass a broken Array.prototype.toJSON method. This lead to a broken Array.prototype.toJSON method for all users. Should get fixed soon.