During mid March something changed on Youtube which results in videos showing flash player for an instant (a few seconds with slow connection) then falling back to html5 player http://www.youtube.com/watch?v=nqihx_K-Y-o # uses html5 player !!! http://www.youtube.com/v/nqihx_K-Y-o # uses flash player http://www.youtube.com/embed/nqihx_K-Y-o # uses flash player (iframe api) or displaying the following message The Adobe Flash Player is required for video playback. The issue doesn't happen when flash plugin is pushed out-of-process only by plugin-container or only by npviewer.bin. It's when they're both used as I've managed to reproduce on ubuntu. However, the only choice to have flash on BSDs is to use nspluginwrapper as there's no native plugin. And now youtube wants dom.ipc.plugins.enabled -> false while every other flash site works fine without.
You need to contact youtube about this: there is nothing that we can effectively do in-product.