Multiple checkboxes check as in Gmail
For some reason the tag is working differently when highlighting text in Firefox. I think the reason it is failing is because when you hold down shift and click on the label, it doesn't check the box and thus doesn't fire the labeled event.
Check out this: http://jsfiddle.net/xerf/Prxdn/10/
This works both in webkit and FireFox. I changed the labels to span tags. With a bit of CSS, you can fix the padding.