Note: This feature is currently only available on Firefox 6 Aurora (6.0a2).

A new feature coming in Firefox 6 (scheduled for release on August 16th) is a about:permissions.

About:permissions, like about:config, add-ons, and just about every aspect of Firefox, is all about providing the most highly-customizable browsing experience possible. Type about:permissions into the address bar and you’re presented with a dashboard that lets you configure password retention, geolocation, cookies, pop-ups, and offline storage access on a per-site basis.

Source: ExtremeTech