I have just updated to v2.4 my extension FullerScreen for Firefox 3 on addons.mozilla.org and it should be available there soon. Fullerscreen is an enhancement to the FullScreen mode of Firefox, and offers support for CSS "@media projection", allowing to write slideshows in HTML. FullerScreen also adds navigation widgets for the slideshow mode. Read more about FullerScreen here (and that doc is itself a slideshow).