add note to future

packages/core/clockworker.js

@@ -1,5 +1,8 @@
 // eslint-disable-next-line no-undef
 importScripts('./neozyklus.js');
+// TODO: swap below line with above one when firefox supports esm imports in service workers
+// see https://developer.mozilla.org/en-US/docs/Web/API/ServiceWorker?retiredLocale=de#browser_compatibility
+// import createClock from './zyklus.mjs';
 
 function getTime() {
   const precision = 10 ** 4;