Bug 847168 - [User Story] Email Signature

I suspect you're trying to disable this to work around the label element working in your favor.  Specifically, the label magically hooks itself up to the checkbox.  What I believe you want to do is add your click listener on the input node itself and do not disable anything.  Then clicking on the checkbox triggers the click on the checkbox, but so does clicking on the label.  If the label didn't directly contain the input, you could use the "for" attribute.  See http://www.whatwg.org/specs/web-apps/current-work/multipage/forms.html#the-label-element