The problem seems to have started after a browser update. Running up-to-date Windows 10. This issue does not occur when using Edge. Youtube videos are blank (spinning). Sometimes it results is a still frame, other times it spins forever. Many times, you can see a series of stills in the video as the time line is dragged from the beginning to the end of the video. A small number of videos will work, but I have not been able to predict which will work or why. A different Win 10 machine on the same network is fine. I have tried all of the solutions I could find searching on-line (clearing cache, cookies, reboot, restart, update browser software, etc, multiple times). Checked settings in windows, firefox and youtube looking for something. Any help would be appreciated. Have used Firefox for decades and I don't want to be forced to change browsers.