"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

Commencez par les instructions du mode attach dans la README et reproduisez la configuration avec un paramètre global incorrect de firefox.executable. Suivez la manière dont le lancement en mode attach lit ce paramètre et produit l’erreur ; le travail est terminé lorsque le mode attach ne nécessite plus d’executable inutilisé, ou lorsque le message identifie clairement le paramètre global de l’extension.

Rédigé par le modèle d'indexation à partir du texte de l'issue.