Jetpack - How to stop a contentscript setInterval?

data_contentscript.js

self.on("message", function(tab_id) {
	console.log("Starting loop for: "+tab_id);
	setInterval(function() {
		console.log( "interval "+tab_id+": "+(new Date()).getTime() );
	}, 1000);
});

Explanation

It seems that the content script of a page-mod continues to execute even when a tab is closed/detached.

As you can see in this example output, I opened two tabs. Every second the content script outputs the tab_id received and the current time. I only closed the second tab. I have two problems:

1 - I don't understand why the "Detached a tab" message appeared twice while only one tab was closed. Anybody can explain this behavior or confirm that it's a bug?

2 - The two intervals that were started in the content script continued to run. I expected that the content script would stop executing completely when a tab was closed/detached. How do I make sure that the content script stops executing completely when a tab is closed/detached?

lib_main.js

var pageMod = require("page-mod");
var data = require("self").data;

var mod = pageMod.PageMod({
    include: "*",
    contentScriptWhen: "ready",
    contentScriptFile: [ data.url("contentscript.js") ],
    onAttach: function(worker) {
		worker.postMessage(worker.tab.index);
		
		worker.on("detach", function() {
			console.log("Detached a tab.");
		});
	}
});

output

info: Starting loop for: 0
info: interval 0: 1314234077833
info: interval 0: 1314234078834
info: interval 0: 1314234079836
info: interval 0: 1314234080835
info: Starting loop for: 1
info: interval 0: 1314234081828
info: interval 1: 1314234081915
info: interval 0: 1314234082830
info: interval 1: 1314234082916
info: interval 0: 1314234083848
info: interval 1: 1314234083909
info: Detached a tab.
info: Detached a tab.
info: interval 0: 1314234084847
info: interval 1: 1314234084898
info: interval 0: 1314234085843
info: interval 1: 1314234085896