Constant HTTP 403 for Console Frame in MDN Playground

Summary

Issue Details

Recently, every time I go to use the Playground via MDN tools, I am presented the following message for the page loaded in the Console frame of the Playground page. Note that subdomain and search params are different each time I open the Playground, but the gist of the error is always the same.

This is not an issue using Firefox Developer Edition nor Safari - a blank page is loaded in the Console frame as expected. I do not use Chrome, Edge, etc. so I have not tested those browsers.

I am logged into my Mozilla account, all benefits of my paid MDN subscription enable other functionality as expected, and I can access other paid Mozilla services without issue.

Normal troubleshooting has been performed - clear cache, cookes, etc.; try with add-ons disabled, try with a new profile. I do use VPN but the issue is the same with and without VPN. I do use uBO, LocalCDN, and CanvasBlocker, and they are all setup to leave all Firefox and Mozilla domains alone.

Error Message

Looks like there’s a problem with this site

https://51cb7e78-56e0-4f96-82e8-575f7503bb19.mdnplay.dev/runner.html?uuid=51cb7e78-56e0-4f96-82e8-575f7503bb19&state=q1bKKMnNUbJSUtJRSi4uhjCyoHRJRmpuKpCZkliUrVQLAA== sent back an error.

Error code: 403 Forbidden

Check to make sure you’ve typed the website address correctly.

URL

https://developer.mozilla.org/en-US/play

Reproduction steps

1. Launch Firefox [currently 142.0.1 (aarch64) on macOS 15.6.1 (24G90)]
2. Go to https://developer.mozilla.org
3. Click the "Tools" menu
4. Select the "Playground" item in the left half of the menu
5. Playground opens and its Console frame displays the aforementioned error

Note: The issue is the same when going directly to https://developer.mozilla.org/en-US/play.

Expected behavior

When opened, Playground should be usable without an error page loaded in its Console frame.

Actual behavior

When opened, the Playground page is loaded with the Console frame displaying the aforementioned error.

Device

Laptop

Browser

Firefox

Browser version

Stable

Operating system

Mac OS

Screenshot

Anything else?

No response

Validations

• I have read the Community Participation Guidelines.
• I have verified that there isn't already an issue that reports the same bug to avoid creating a duplicate.
• I have checked that this is a concrete bug. For Q&A open a GitHub Discussion.

Start at the Playground route https://developer.mozilla.org/en-US/play and reproduce the Firefox failure in the Console frame. Inspect how the frame requests the mdnplay.dev runner URL; done means the Playground opens with a usable Console frame instead of a 403 page.