I can't click on the icons at the top of a site. What's causing this?

I have firefox and it's having a strange problem today. When I'm on facebook, I can't click on the icons at the top. I can't click the message, friend request, notifications, home, profile icons and can't click the one that allows you to sign out. So i can't even sign out. What I mean by can't click is when I put the cursor over those icons, it doesn't change into the finger. When I left click it anyway, nothing happens. This doesn't happen in internet explorer, but I want to keep using firefox. What's causing this? I also noticed that this doesn't just happen on facebook. I've uninstalled and reinstalled firefox. I restarted the PC. It didn't help.