[Firefox] Markdown Preview fails to load workspace CSS via markdown.styles (NS_ERROR_UNKNOWN_HOST for vscode-resource.vscode-cdn.net)

[belonesox]

Is there an existing issue for this?

• I have searched the existing issues

OS/Web Information

• Web Browser: Firefox
  • firefox-151.0-2.fc42.x86_64
• Local OS: Fedora 42
• Remote OS: Fedora 42
• Remote Architecture: amd64
• code-server --version: 4.105.1 811ec6c with Code 1.105.1

Steps to Reproduce

Bug Description

When opening a Markdown preview in Firefox, local CSS files referenced in markdown.styles fail to load. The browser attempts to resolve the virtual vscode-resource.vscode-cdn.net domain via DNS instead of intercepting it via the Service Worker, resulting in NS_ERROR_UNKNOWN_HOST.

This issue is specific to Firefox and its implementation of Service Worker / iframe isolation (dFPI).

Looks close to #7892 … BUT!!! The same setup works for me perfectly in Chromium-based browsers.
So I hope some workaround (custom settings, hacks) exists that can make work it with Firefox.

Steps to Reproduce

1. Run code-server and access it via Firefox.

2. In a workspace, create a file test.css in the root directory.

3. Add the following to .vscode/settings.json:

   {
       "markdown.styles": ["test.css"]
   }

4. Open any .md file and open the Markdown Preview.

5. Check the developer tools Network and Console panels.

Expected

The Service Worker intercepts the request for test.css and serves the local workspace file, applying the custom styles to the preview (as it does in Chromium).

Actual

The preview renders without the custom CSS. In the Developer Tools Console:

Could not load 'markdown.styles': test.css

In the Network panel, the request for test.css fails:

• Host: vscode-remote+<punycode>.vscode-resource.vscode-cdn.net
• Error: NS_ERROR_UNKNOWN_HOST
• DNS Resolution: System

Investigation Details

Based on HAR exports and console logs:

1. The Service Worker is registered and active.
2. Extension resources inside the webview (e.g., index.bundle.js from extensions) are successfully intercepted and return HTTP/2 200.
3. However, the dynamically injected <link rel="stylesheet"> for markdown.styles bypasses the Service Worker and goes directly to the network.
4. Setting privacy.partition.serviceWorkers to false in about:config completely breaks code-server initialization (Error: IndexedDB database 'vscode-web-db' is closed), so it cannot be used as a workaround.

It appears to be a race condition or a cross-origin iframe isolation bug specific to Firefox, where the Service Worker fails to control the CSS fetch request initiated by the Markdown preview webview.

Attachments

Please find the attached console.log and .har (har.zip) network export demonstrating the bypassed Service Worker.

console-export-2026-7-30_12-31-17.log

xn--80ach2ae.xn--80apqgfe.xn--p1ai_Archive [26-07-30 12-32-00].har.zip

Logs

Screenshot/Video

No response

Does this bug reproduce in native VS Code?

Yes, this is also broken in native VS Code

Does this bug reproduce in VS Code web?

Yes, this is also broken in VS Code web

Does this bug reproduce in GitHub Codespaces?

Yes, this is also broken in GitHub Codespaces

Are you accessing code-server over a secure context?

• I am using a secure context.

Notes

No response

[linear-code]

DEVEX-701