Sorry to bump this up but i am getting this problem for a couple weeks now with FIREFOX. I've seen some new threads about this on the firefox forum and i believe it's related to this problem. Suggestion of clearing cache etc doesn't work.
If i add the
you can see that basically the ad is hidden but it seems to have to wait for the length of the ad before showing the selected content. Here is a screenshot, you can see the bottom left corner showing that the ad it's supposed to be running.
Again this only happened if you have ABP + LOGIN, if logout there is no problem.http://s2.postimg.org/tgqg7rgqh/youtube_AD.jpg