Now with this title I am not saying that you should throw away JQuery etc… not at all… Lord knows I never will. But Mozilla has created a lighter version of JQuery to assist with Mozilla OS development and mobile HTML5 app development in general. When you download Mortar to have a play you will notice it is included.
Zepto Home Page http://zeptojs.com/
Writing Web Apps with Mortar https://hacks.mozilla.org/2013/01/writing-web-apps-quickly-with-mortar/