On some sites, the video controls flash intermittently while playing. On one in particular - theguardian.com/uk - embedded video content will not play. The sound will, but the picture won't. I've done seemingly everything - disable hardware acceleration, messed about with tracking, blocking, permissions, extensions, plugins, reset that site, refreshed Firefox. Nothing works. I've been using Firefox happily for over a decade, but this is severely testing my faith and allegiance. I'm using a 2019 Mac, but Apple say it's not their problem. The protracted steps I had to take to even get to this page on this forum have seriously tested my patience. Please, Firefox - how can I fix this?