I have this problem also and created a forum account specially because of that problem.
Problem : On some Youtube videos, when the page loads, the videos stays black with nothing happening and, after 15/30 seconds, the video starts to play.
Configuration : Windows 10 x64, Firefox 44.0.2 x32, Adblock Plus 2.7.2
Possible cause of the problem : I tried several things (including what you said mapx), and I think I found the source of the problem. The problem seems to be related to your Google/Gmail/Youtube account. Each time I log in my Youtube account I have the problem; but as soon as I log out, the problem disappears.
I would like other people, that have the problem, to test this out : simply log out of your Google/Gmail/Youtube account and try on different videos (PewDiePie videos are good to test, because he has "pre-video ads" on almost all his videos).
Please keep us informed.