Allow the user to right mouse click on an address in the address bar and add a "BLOCK" option in the drop menu that automatically appears. When the user clicks on it, offer a second chance "ARE YOU SURE" consent box with a YES / NO buttons just in case someone right clicks by mistake. Clicking NO obvious will stop the block option all together. Clicking YES will tell Firefox that you no longer want this website to appear in their browser. An additional option with clicking YES would be a write in reason for blocking, ie: spam, virus, unwanted pop-up, etc. This could aid in categorizing the reasons for blocking and ease of research.
In this age of popup windows/tabs that Firefox is UNABLE to prevent with their useless pop-up blocker, this is the next step in prevention. This option would also help prevent malicious viruses from getting through to a person's computer as many viruses are tied to addresses. Also Firefox should monitor these addresses for repeat blocks by multiple users. If one address is blocked by everyone then that address should be permanently block by Firefox itself without further need from users. If one address is blocked by 5 out of 100 users then no action should be taken by Firefox to permanently block that address.
Since Firefox allows each user to customize their browser, IE: Bookmarks, Add-ons, Themes, etc. this option could be attached to that customization in the same way. This will allow each user's browser to still be their own.