I don't think there is a way to "favorite" a thread other than bookmarking it in your browser.
You can, however, "Subscribe" to a thread. When you do this, there is always a link to it in your User Control Panel. You can set options to have an email sent to you if someone adds to it.
To Subscribe, click on Thread Tools (at the top of the thread, not the individual post) and select "Subscribe to this Thread". You'll be presented with a list of options on notifications, etc.
Have Fun!
:cheers:
Wade