Delay for debugger to attach

I'm experiencing a delay between launching the debugger in VS Code and having the debugger fully attach. The browser opens, and the page loads, but all of the breakpoints in VS Code are grayed out and the context menu in the browser won't open. After about 30 seconds, the debugger finally finishes attaching, the breakpoints become active, and the context menu starts working in the browser. I'm on a Mac with the latest version of iOS, Firefox and VS Code, using the following config;
```
{
"name": "Launch Firefox",
"type": "firefox",
"request": "launch",
"url": "http://localhost:3000",
"pathMappings": [
{
"url": "http://localhost:3000/path/to/my-repo/",
"path": "${workspaceFolder}/"
}
]
}
```

Any ideas what might be causing this delay? Has anyone else experienced this? I'm happy to help debug further, but I'm not sure where to start looking, so any hints would be helpful.