When browsing the commits in a pull request, there is no direct way to go back to the pull request. The back button of your browser only works if you did not clicked into deeper commits using the 'parent rev' / 'child rev' links.
In general, there is no connection between commits and the pull requests they are part of, nor is there a connection (AFAICS) between comments on a commit and the pull request it was made in.
I think such a connection could prove useful. At a minimum, we should add a link from commits to their containing pull request.