Debugger and current debugging line are both misplaced

This is the only thing currently keeping me from switching to firefox

[Repo to reproduce](https://github.com/ldiane/vfd)

Repro steps:
```
add launch.json from below
npm install
npm run dev
In src/App.vue add a breakpoint between line 5 and 8
Start debugging
```

I am trying to debug from vscode a vue 3 with TS and vite project

As stated in the [docs](https://marketplace.visualstudio.com/items?itemName=firefox-devtools.vscode-firefox-debug) i did:

> open the Developer Tools Settings and check the checkboxes labeled “Enable browser chrome and add-on debugging toolboxes” and “Enable remote debugging”

and executed Firefox as follows

`"C:\Program Files\Mozilla Firefox\firefox.exe" -start-debugger-server`

The problem is that the debugger is all jumpy and doesn’t hit the correct line:

Here is my launch.json (the one for chrome works perfectly well)

```
{
"version": "0.2.0",
"configurations": [
{
"type": "chrome",
"request": "attach",
"name": "Attach to Chrome",
"port": 9222,
"webRoot": "${workspaceFolder}",
"urlFilter": "http://127.0.0.1:4000/*"
},
{
"name": "Attach to Firefox",
"type": "firefox",
"request": "attach",
"url": "http://127.0.0.1:4000",
"webRoot": "${workspaceFolder}",
"pathMappings": [
{
"url": "http://127.0.0.1:4000/src",
"path": "${workspaceFolder}/src"
}
]
}
]
}
```

リンクされている vfd リポジトリとその launch.json を使用して問題を再現します。npm install、npm run dev の順に実行し、次に src/App.vue の 5 行目と 8 行目の間にブレークポイントを設定して Firefox 構成を開始します。デバッガーの位置と現在のデバッグ行を、動作する Chrome 構成と比較します。Firefox が正しいソース行で停止すれば完了です。

索引モデルが issue の本文から書いたものです。