Only began happening recently. The audio continues but the video freezes. Can reload the page, but then it always does the same. It's fine on Chrome. Am using the latest versions of both browsers on Windows 10.
It's the exact same video served to both Firefox and Chrome, so it would suggest a browser issue. Could possibly be a browser extension causing it? Or maybe an update to Firefox itself that has reduced performance. Try running Firefox in safe mode to see if it still occurs ... if it doesn't then disable extensions one by one to find the culprit.