In addition to its thumbnail optimizer, the add-on includes a simple slideshow viewer that doesn't attempt to replace Flickr's own gorgeous slideshow tool. Instead, it simply fades together the pre-fetched pictures without leaving the page. You can set how long you want each photo to be displayed, anywhere from one to five seconds.
If you're a heavy Flickr user this extension is definitely worth the download as it will cut your page loads in half. It retains Flickr's slideshow tools and its same navigation; you also have the option to turn it on and off from a setting at the bottom of your browser.
Note: As with all experimental Firefox add-ons, you must be registered with Mozilla's Firefox add-ons site to download it. It's also available as a Greasemonkey script.

Flickr Gallery Plus lets you go back and forth between shots with your keyboard's arrow keys. It also pre-fetches each image--letting you load it without the page re-loading.