Firefox 72.0.1 64-bit on MacOS Catalina 10.15.2. The issue start yesterday (2020-01-15). Youtube video start frequent pausing for buffering, which I don't experience for a long long time. In the past, only 8K videos require a bit longer start time. 4K and below are quite responsive when I move back and forth. However it changed starting yesterday. I tried with Chrome and Safari with "stats for nerds" and notice a huge difference on connection speed between Firefox and the other 2 browsers. Attached result in following order: Firefox, Chrome, Safari. I tried multiple videos and results are similar.