BlockSite is an extension, which automatically blocks websites
of your choice. Additionally, this extension will disable all
hyperlinks to these websites, by just displaying the link text
without the clicking functionality.

To download and install the FireFox BlockSite Add-On into your FireFox Browser, Click Here!

From that page, click on the "Add to Firefox" button, then when the dialog box appears, click on the "Install Now" button.
Once, the button is clicked and the BlockSite add-on is installed, close all Firefox windows and restart the browser.

To add sites you want to block to your blacklist, from FireFox's menu-bar, click Tools, Add-ons, look for the BlockSite plug-in then click on "Options". From there you can now easily add any sites you want to block into your blacklist.

I find this works great for blocking certain advertisement related websites and their content that sometimes try to force their ads on you when visiting certain sites. For example; lately when visiting Yahoo, I was being blasted with a bunch of ads that was being loaded from an external site (not hosted by yahoo), but sometimes I found their ads were greatly slowing-down or even sometimes crashing my browser with bad scripting errors. I don't mind seeing ads most times (we come to expect it from browsing the internet, and it's how some sites can manage to stay afloat), but when they greatly slow-down my browser to a crawl or cause crashes, I don't like that. Since I added that particular site that was delivering those problem ads, I can now browse through Yahoo again without those problems being caused by the ads from that particular site.