Catch xhr request with new js files

Hi,

Currently we're using twig for get some js files for the views. But the debugger isn't catching the new files, and the breakpoints are always in gray.

Firefox dev version: 82.0b9 (64-bit)
Vscode version: 1.50.0

twig request code:
```

{% if javascript|length %}
{% for script in javascript %}

{% endfor %}
{% endif %}

```

launch.json
```json
{
"name": "Listen for Firefox",
"type": "firefox",
"request": "launch",
"reAttach": true,
"profile": "dev-edition-default",
"url": "http://localhost/",
"pathMappings": [
{
"url": "http://localhost/",
"path": "${workspaceFolder}/"
}
],
"skipFiles": [
"${workspaceFolder}/**/*.min.js"
],
"log": {
"fileName": "${workspaceFolder}/logs/firefox-debug-log.txt",
"fileLevel": {
"default": "Debug"
}
}
}
```