At this time it seems the "anti adblock feature" is being rolled out casually. You might trigger it, you might not. I have FF+uBlock Origin and still saw it, pretended I was about to file a bug report on it, dismissed the pop-out, then got to see the video ad-free. I suspect this will not be the case a week or so from now.
Not only this, but sometimes the RAM usage spikes if I open Youtube in Firefox with uBlock enabled. I have noticed this only for this combo "Youtube + FF + uBlock" on my Mac, specially in last week. Before that I rarely saw my FF freezing.