This extension was the object of a TorrentFreak article recently. It uses the libtorrent library, integrates itself to Firefox's default download manager, and is currently in alpha phase. There's no preferences dialog - your downloads are saved to the Desktop - and upload is hard limited to 15kB/s. Here's an screenshot and announce dump:

Started:
Code:
GET /announce?info_hash=*&peer_id=-LT0E00-YWq*BOXt_il.&port=6881&uploaded=0&downloaded=0&left=728221696&event=started&key=e86ffd97&compact=1&numwant=200&ipv6=&no_peer_id=1 HTTP/1.0
Host:torrent.ubuntu.com
User-Agent: libtorrent/0.14.0.0
Connection: close
HTTP/1.0 200 OK
...
Stopped:
Code:
GET /announce?info_hash=*&peer_id=-LT0E00-YWq*BOXt_il.&port=6881&uploaded=0&downloaded=316371&left=728221696&event=stopped&key=e86ffd97&compact=1&numwant=0&ipv6=&no_peer_id=1 HTTP/1.0
Host:torrent.ubuntu.com
User-Agent: libtorrent/0.14.0.0
Connection: close
HTTP/1.0 200 OK
...
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