YouTube Center
If you watch YouTube frequently or occasionally, and you have not a very fast internet connection, YouTube Center surely is what you will like.
Quite a long time, I found that YouTube does not work like before. I cannot pause the video and wait downloading the video completely then only watch it. Now, the video will download a while then stop downloading. Besides that, navigate to different time frame will need to re-download the video. As a result, I cannot watch the video freely after downloading to the web browser.
But using this YouTube Center, now I can disable the Dash Playback (as mentioned above). Now watching YouTube makes me feel easier.
YouTube Center is actually a userscript. However, there is also the Firefox extension, Google Chrome extension, Opera extension, etc.