Why Firefox subscription bell button does not show up?
Such subscription button behaviour is related to the logic of a system browser subscription pop-up.
When you click on the "Not now" option, the push permissions stay in the default state and Firefox would not allow asking for push subscription second time in the row.
At the moment there is no workaround available, it is just the part of a Firefox logic. However, it is worth to note that in case if you refresh the page - the bell button will work as supposed to.