Backspace forces cursor to end of text (Firefox)

### Repro steps

Using Firefox 124/125, visit https://medv.io/codejar/, focus in the editor and delete all the code.

Enter the following code:

```javascript
const greet = (name) => {
console.log(5);
}

greet("World");
```

Place the cursor after the `5` and hit the Backspace key on your keyboard.

### Expected

Deletes `5` and cursor remains between parentheses.

### Actual

Deletes `5` and cursor jumps to the end of the file.

### Environment

Firefox 124/125, Windows/macOS.

Does not reproduce in Chrome or Safari.