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

### Is there an existing issue for this?

- [X] 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 811ec6c1d60add2eb92446161ca812828fdbaa7f 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 coder/code-server#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`:

```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:

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

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

* **Host**: `vscode-remote+.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 `` 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

```shell
```

### 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*

Reproduce the Markdown preview with markdown.styles set to test.css in Firefox, then inspect the Service Worker registration, Network panel, and console for the vscode-resource.vscode-cdn.net request. Done means workspace CSS is intercepted and applied in Firefox without breaking code-server initialization or IndexedDB.

Written by the indexing model from the issue text.