I really don't think that such keyboard shortcut exists. The reason being that absolutely most modern sites will not let you just run back and forth like that... you will simply get access denied error response. It still could make some sense when you actually ftp using the browser. Now the "parent key" actually may help alot. I know that this isn't the kind of help you require but well :)
Oh wow! I think i found what you need for Chrome:
https://chrome.google.com/webstore/detail/navigup/eigojmmlmgigbldiclbpibmjnlbdpiaa
And similar one for Firefox but very old...
https://addons.mozilla.org/en/firefox/addon/go-up/
I still don't know where it could be useful but seems like there is a demand for it :)