更新：执行定时器结束操作。
您需要在if条件中添加逻辑来执行操作。

if (counter == firmwareList.length) {
    timer.cancel();
    developer.log('This comes afterwards'); //here
      }
});

下面是一个微件示例

class TimerF extends StatefulWidget {
  const TimerF({super.key});

  @override
  State<TimerF> createState() => _TimerFState();
}

class _TimerFState extends State<TimerF> {
  var counter = 0;

  Timer? timer;

  bool isTimerActive = true;

  @override
  void initState() {
    super.initState();
    timer = Timer.periodic(const Duration(seconds: 1), (timer) {
      print('Still timer');

      if (counter == 5) {
        timer.cancel();
        isTimerActive = false;
      }
      counter++;
      setState(() {});
    });
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          Text("current value ${counter}"),
          if (timer?.isActive == false) Text("Timer isnt active"),
          if (isTimerActive == false) Text("Timer isnt active $isTimerActive"),
        ],
      ),
    );
  }
}

我在当前代码片段中看不到counter上的任何增量。这就是为什么counter保持为0并且不满足取消计时器的条件。
也许会是

var counter = 0;
Timer.periodic(const Duration(milliseconds: 50), (timer) {
  print('Still timer');

  if (counter == 5) {
    timer.cancel();
  }
  counter++; //increment the value
  print('this is afterwards');
});

下面是一个使用Timer的完整示例

Timer? timer;

startTimer() {
  var counter = 0;
  timer = Timer.periodic(const Duration(milliseconds: 50), (timer) {
    counter += 1;
    print('Actual step $counter');
    if(counter==5) {
      cancelTimer();
    }
  });
}

cancelTimer() {
  timer?.cancel();
  print('this is afterwards');
  //do something here
}

当您startTimer()时，输出如下

Actual step 1
Actual step 2
Actual step 3
Actual step 4
Actual step 5
this is afterwards