A linkbar button with some Javascript behind it is a lot easier than clicking into the URL, highlighting text, deleting it, hitting Go or Enter, and so on, over and over again. Here's the Javascript that will do this (prefaced by "javascript:" so that it'll run in the address field of the browser):
javascript:ar=location.href.split('/');
if(ar.pop()=='')ar.pop();
u=ar.join('/');
location.href=u;
Remember that for this to work as a bookmarklet, it all has to be on one line. I've broken the code apart here for illustration purposes.
All we do is make array out of the individual location elements of the current URL by breaking it at forward slashes, then pop the tail element off, re-join() the array with '/' delimiters, and make the browser go to the newly formed URL.
Works like a charm.
I keep this script in a link button (called "Peelback") on Firefox's linkbar. It's handy as heck when you've landed on an interesting web page and you want to further navigate a given URL via the ancestor axis.