Another extension issue as noted at https://support.mozilla.org/en-US/questions/1063428
Or it's just borked by Facebook. If it's just Facebook you deal with their support as there is nothing on your end to tweak.
I had embedded many Youtube videos to my Facebook timeline and all of them had play buttons before enabling the videos to be watched on the Facebook timeline itself. Just a few days back all the play buttons were suddenly missing from the embedded Youtube videos and clicking on any of the images of the videos would bring me right to the Youtube website in order to watch it. Does anyone know how to troubleshoot this problem so that the play buttons will reappear on all the Youtube videos posted to my Facebook timeline? Thanks a lot for any advice relating to this matter.