Hey pal..
What happens when you try to open these sites?? Is there a specific error message??? Or may be some plug-ins missing?? Sometimes some malwares also restricts access to specific sites.. You can install malwarebytes and run a scan. See if that helps...
I've suddenly found that my Firefox browsers no longer works with YouTube, Facebook, etc. in the past few days. Is there something going on that I don't know about?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic