there isn't any option like that for firefox, you`ll have to do it via the HOST file.
it`ll be here:
then click the host file a box will appear asking what app to open it in. Choose notepad.
Then at the bottom where it has this:
Add your websites you wish too block such as:
then click file then save not save as do not choose to save it as a .txt file just click save, those websites should be blocked then.
EDIT: ok I found the above solution may not work maybe I did it wrong
you can also block websites using your firewall.