How to selectively clear Firefox history
But if you want the same functionality without upgrading to a beta version of Firefox, you can check out the Close 'n forget add-on for Firefox 2 or 3.0.
You can configure Close 'n forget to to remove cookies from a web site when you forget it. And you can even have it delete every page you've visited in a browser tab, which makes the add-on a bit more powerful than the built-in Firefox 3.5 feature.
[via Mozilla Links]