There is a button at the bottom of the browser window that is a 3-way toggle. It looks like two arrows chasing each other's tails. Press it repeatedly to change it's setting.
When it is gray, the playlist will play to the end then stop.
When it is blue, the playlist will re-start at the beginning after it finishes the last song.
When it is blue and has a little "1" in it, it will repeat the current song.
In the below screen shot, it is the second to last button: