Can't play video or video plays with no sound on certain webpages only

Hello, about 1-2 weeks ago a very strange problem occurred - on some pages videos don't play at all or play without a sound. Can anyone help me with this strange situation? For example, every video on youtube.com plays without a problem; on facebook.com every video plays with no sound at all; on some other pages video doesn't play at all. This problem is only with firefox, if I try to open the same videos with another browser everything is fine.