"Couldn't find the Firefox executable" when launching in attach mode

I am getting the `Couldn't find the Firefox executable` when launching in attach mode. I am asking myself now if this is a wanted behaviour, since I wouldn't expect that the executable is needed for that.

## Background
I was following the instructions for launching in `attach` mode [in the README](https://github.com/firefox-devtools/vscode-firefox-debug#attach).
Accordingly, I have set up a `launch.json` that contained the recommended block:
``` json
{
"version": "0.2.0",
"configurations": [
{
"name": "Attach to localhost",
"type": "firefox",
"request": "attach",
"url": "http://127.0.0.1/"
}
]
}
```
My firefox had the necessary `about:config` changes and launched fine in debug mode with
``` bash
"C:\Program Files\Mozilla Firefox\firefox.exe" -start-debugger-server
```
When I launched the debug session in vscode with this configuration I got the `Couldn't find the Firefox executable` error.

## Cause
The `firefox.executable` config was set globally to a wrong location.

Two things that I think would have improved my situation
- the error message would point out that this is set in the global extension settings
- the error would not be thrown since (correct me if I am wrong) the executable is not called at all in `attach` mode

Comienza con las instrucciones del modo attach en la README y reproduce la configuración utilizando un ajuste global incorrecto de firefox.executable. Traza cómo el lanzamiento en modo attach lee ese ajuste y produce el error; se considera terminado cuando el modo attach ya no requiere un executable no utilizado, o cuando el mensaje identifica claramente el ajuste global de la extensión.

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