Firepuddle is a project to build support for Bittorrent into Mozilla Firefox as an extension. It is part of Google’s Summer of Code. An alpha release of the extension is due 31st August (this Wednesday) with a Beta due in 2 weeks.
At the moment the extension still lacks .torrent integration with Firefox, only allows one concurrent download and a single tracker connection and doesn’t allow resuming across different sessions. I expect these issues will probably be resolved for the beta or final version.