I have also tried clearing the cache and having adblock off. Interestingly, with adblock off, I will see the video for the ad, but not the video for the youtube video.

So I'm guessing it has to deal with the streaming tech used for youtube videos.

This could be a problem with the HTML5 media player that is fixed with a restart.

Did you interrupt or paused any videos?

Does this also happen if you use the Shockwave Flash player on YouTube?

• https://addons.mozilla.org/firefox/addon/youtube-flash-video-player/

The problem will disappear after restarting firefox... until I've watched some more youtube videos and it stops showing videos again. It's annoying to have to restart firefox after every 5th video.

Video files and plain html5 videos on other websites work when this problem occurs.

I haven't had flash installed for several years.

I'm having the same problem, but it just started today. Previously I wasn't experiencing this issue. I'm using Firefox 50.1.0 for Mac. I do not have Flash installed.

Also, I haven't installed anything new recently or changed any settings. The problem simply started this morning for no apparent reason.

Bug still present in firefox 51. Restarting firefox doesn't solve the problem anymore. Clearing the cache "fixed" it. this time.