Opera comes with an inbuilt torrent downloader which allows you to download torrents from the internet. Although the same functionality doesn't come inbuilt in firefox, there are couple of extensions available for firefox which allow you to download torrents.
Firetorrent was initially created for a browser named wyzo, a firefox modification like flock. It is designed with people having minimal bittorent experience in mind.
Firetorrent adds a seperate tab to the browser's download manager where you can see all the torrents which are downoading. It is the best and most easy-to-use torrent downloading extension available today for firefox.
Foxtorrent integrates nicely into firefox and starts downloading the torrent as soon as you click on any torrent link. However, we believe that it lacks usability. As it is still in beta this might be an extension to look out for in future.