The only way the MAC address can be found through a browser is by running Flash, Java or ActiveX code that collects and sends it. Flash and Java you shouldn't be using at all, and ActiveX is only supported on Internet Explorer, not to mention it triggers lots of security warnings. The same is true of your computer name.

One detail that can be read is your local IP, if you haven't disabled WebRTC or secured your browser accordingly. However, that is not nearly as specific as a (theoretically globally-unique) MAC. However², the concepts of "presumption of innocence" and "circumstantial evidence" do not exist in the torrent world, so you should still take measures to protect yourself against this.