Important: Personally, I started using https://addons.mozilla.org/en-US/firefox/addon/bmreplace/
as it works much better than this one (I never got it to work on the sidebar). I suggest trying it, too.
Unfortunately, the original UpdateBookmark Addon ( https://addons.mozilla.org/de/firefox/addon/update-bookmark/
) hasn't yet been ported to Firefox 4 or 5, so I wrote a simple (and much less powerfull) replacement until it gets done.
This addon simly adds an "Update" button to the context menu of the bookmarks. Clicking it updates the addrerss of the bookmark to the currently displayed page. No checks are done at the moment, so be carefull not to replace your important bookmarks with about:blank :-)