overlay0: try & examine wiz-generated overlay, talk about overlays and insertion points, look at js and css; ask students to experiment
overlay1: simplify xul, add sidebar snippet pointing to about:mozilla (don't explain broadcaster, treat as snippet)
overlay2: point to custom sidebar and create sidebar0

sidebar0: hello world. a separate toplevel
sidebar1: add simple interaction
sidebar2: separate UI and behaviour
sidebar3: introduce vertical layout and center alignment
sidebar4: introduce horizontal layout and dom addressing (getElementById)
sidebar5: introduce DOM manipulation (node creation and event handler addition)
sidebar6: introduce AJAX calls from chrome
sidebar7: raw twitter search from sidebar
sidebar8: example of more complex widget