Firefox has no built-in option for blocking sites.

A few options for you.

1. Install the FoxFilter add-on, the downside is add-ons are fairly easy to circumvent - https://addons.mozilla.org/firefox/addon/foxfilter/
2. Another add-on that can filter content is the ProCon Latte Content Filter - https://addons.mozilla.org/firefox/addon/procon-latte/
3. To block specific sites you can use the BlockSite add-on - https://addons.mozilla.org/firefox/addon/blocksite/
4. Install a 3rd-party parental control program such as the free K9 web protection - http://www.k9webprotection.com/
5. Use the OpenDNS service, there is a free version, and with it you can filter content based on categories that you choose - http://www.opendns.com/

I recommend either of the last 2 options, add-ons are quite easy to bypass.

Another method is to use the HOSTS file, which is part of almost every operating system, to "loop" specific web addresses back into your PC and prevent any content from those addresses from being loaded. It is mostly used to block advertisements and other undesired or malicious content.
http://en.wikipedia.org/wiki/Hosts_%28file%29
http://windowsdevcenter.com/lpt/a/4730
http://www.internetinspiration.co.uk/hosts.htm

Pro:

• Doesn't require the installation of anything, it is there already, and there is no cost.
  
• It is well hidden in the operating system files, impossible to access without Admin privileges.

Con:

• Needs a certain element computer operating system knowledge, or a bit of reading to learn about it and how to implement the blocks or re-directs.
  
• Blocks web addresses for every user account on that PC, for every browser installed on that PC, and even web browsers on the USB key that is plugged in to that PC.
  
• Too easy to forget about when you are having problems reaching one of your own favorite websites, when they have "moved" the website.

That was one of my first "senior moments". I used it to specify the IP address to be used for a specific forum years ago for quicker loading of forum pages, and when that forum was moved to a different server with a different IP address, I couldn't load it at all. It took me a couple of hours to realize that I had the URL pointed to the old server in the HOSTS file.

The HOSTS file can be used for a number of different purposes - you could even have it display your own "web page" that says, Hey young lady, you know you are banned from YouTube why are you trying to go there?