Can not use env variables in the launch.json

Hello,
I am trying to make debugger configurations pointing to a port specified in the enviromental variables. It works for chrome and for edge but it does not with the firefox debugger.

```
"type": "firefox",
"request": "launch",
"name": "Debug React - Firefox",
"url": "http://localhost:${env:DEVSERVER_PORT}/",
"webRoot": "${workspaceFolder}/react_front/src",
"pathMappings": [
{
"url": "http://localhost:${env:DEVSERVER_PORT}/workspaces/project/react_front/src",
"path": "${workspaceFolder}/react_front/src"
}
]
```
with this configuration it tries to connect to https://localhost instead to http://localhost:3000 (for example), but if I hardcode the port instead it works just fine.

On the other side this configuration works for chrome

```
"type": "pwa-chrome",
"request": "launch",
"name": "Debug React - Chrome",
"url": "http://localhost:${env:DEVSERVER_PORT}",
"webRoot": "${workspaceFolder}/react_front/src",
```

So I don't know if this is a bug, or if there is any workaround to solve this problem. But the harcoded option is really bad because the other devs are going to set their ports and commit them, and CAOS.

Thank you.