Pobox is a Fastmail service, and we use their best-in-breed webmail interface. It uses IMAP so that you can use it with, not instead of, your regular email program, and they should always be up-to-date with each other. All Mailstore accounts have access to Webmail, so you can upgrade your account to try it out.
The main index page uses a preview pane, like most desktop email programs. You can turn this off in Settings, if you prefer to view messages one at a time. It also uses drag-and-drop to move messages between the listing and the various folders (or the Trash.)