The “killer app” for Firefox, AllPeers, is probably the most hyped and anticipated extension out there. It’s set to be released “soon” and TechCrunch got an exclusive preview of it.
Allpeers is a Firefox extension that creates a simple, persistent buddy list in the browser. Initially, interaction with those buddies will be limited to discovering and sharing files – If you choose to, you can share any file on your network with one or more of your friends. They will be able to see what files you choose to share (even getting an RSS feed of new files you include), and with a single click download it to their own hard drive.
Below is a screenshot, but there’s more over at the original article.