Is there such a utility? IE, you add a bookmark in IE7, and it also appears on Firefox 2.0.

Google Toolbar. Though it doesn't save the bookmarks in the browser, it provides what you want.

http://www.google.com/support/firefox/bin/static.py?page=features.html&v=3

PlainOldBookmarks firefox extension adds IE's favourites menu to Firefox – maybe too IE centric for you but at least doesn't attempt to actually sync like some utilities which is probably messy.

https://addons.mozilla.org/en-US/firefox/addon/668

Haven't used it myself but reviews are positive.