Firefox doesn't have an intrinsic 'blocking' of certain sites..
However you can use certain extensions...according to me the 'blocksite' or 'procon' extension comes close to what you are asking about.Go for blocksite.
Click Tools-> Add ons -> make sure you're in the extension tab -> click get extensions at the bottom. In the page which opens up, type in blocksite in the search field.Click on its link when the page opens up.Finally click Install now to install that extension OR click here https://addons.mozilla.org/en-us/firefox... and click install now to install the blocksite extension. Eventually it'll ask you to restart Firefox. After firefox has been restarted, access blocksite's preferences from Tools > Add-ons > Extensions (tab). Here select blocksite and click Options. In options click on Add to add the sites which you want to block and click OK to close the box. Also tick 'enable authentication' and put in a password so that only you can access blocksite's preferences with the password and remove the site from the blacklist if you wish so. Finally click OK to close the preferences dialog box.
Alternatively, you can edit the hosts file. The hosts file will be located in C:\ windows\ system32\ drivers\ etc (provided your windows is installed in C:\windows)
Open the hosts file with notepad or any text editor of your choice.
At the bottom in a separate line in the file enter:
The example line above blocks access to google.com. If a particular site doesn't get blocked, then also enter the site without www and check.For example:
For each site which you want to block do this, that is place an entry on a separate line like i showed you.
However, this won't block access to all the URLs..some nasty ones will slip through...so you'll have to see which ones get blocked..
Anyway, once you alter the hosts file, save it, and restart firefox..and you'll see that the entries which you placed in the hosts file, those sites would not open ...but they would also not open in any other browser which you have, not only in firefox.
Yet another way is by using third party tools for 'parental control'