I haven't found a way in to block websites directly in Firefox but there is a work-around in MS Windows. If you using XP/Vista/7 you can add websites-to-block through the Windows Control panel. Firefox appears to reference to this same list.
Open Control Panel > Internet Options > Security tab > Double click on Restricted Sites (It's the red circle with the line through the middle) > Sites. Add the websites to the list. Click Apply > OK.
Win 7 is Control Panel > Network and Internet > Internet Properties.
I have leechblock and love it. but I have a question about it. It is under tools, under add ons, which is great, but it also shows just on the list under tools and I can't seem to get it to not show there. I do not want kids to know it is on the computer that easily. anyone help with that. I have tried right clicking to delete etc, but I can't find anything that allows me to take it off of the tools menu