The Flash Video Player in YouTube is sometimes better for playing videos on slower computer because the Flash Player seems to use less CPU, and RAM system resources on Firefox. The newer HTML5 video player sometimes uses more CPU, and RAM resources.
The Flash video player also has more screen resolution choices in the settings for the player when you click on the Gears icon on YouTube’s video player.
There is an add-on for Firefox called Firefox YouTube Flash Player which forces YouTube videos to play in YouTube’s Flash video player instead of the HTML5 video player on YouTube. This add-on is helpful if YouTube uses the HTML5 video player by default in Firefox instead of the Flash video player, and you use an older computer which is not fast enough to run the HTML5 video player smoothly, or you want more screen resolution choices like lower resolutions like 144P to save bandwidth/data on a limited connection, or speed up video playback to prevent buffering and pausing on slower internet connections.
After you installed YouTube Flash video player on Firefox, videos would play in the Flash Video Player in YouTube. You’ll also have more screen resolution choices, and better video playback performance when playing back video.
Download YouTube Flash Video Player for Firefox at https://addons.mozilla.org/en-US/firefox/addon/youtube-flash-player/
If you still get the HTML5 video player after installing YouTube Flash Video Player, pressing the refresh button on Firefox, or restarting Firefox by closing, and opening it from your desktop or startmenu should bring up the Flash Video Player in YouTube.
YouTube Flash Video Player currently has 27 five out of five stars reviews from the 28 users who reviewed it on its add-on page on Mozilla.org. Some of the reviews also say that the Flash player on YouTube have better sound/audio quality than the HTML5 version, and videos buffer less, and run better on flash than HTML5 because there are more resolution choices to choose from.
I recommend installing the Latest version of Adobe Flash if you do not have the latest version. You can check if you have the latest version of Adobe Flash by going to http://www.adobe.com/software/flash/about/ .
One of the nice features of using Adobe Flash is that it is easy to turn on and off hardware acceleration by right clicking a flash video, or game, and clicking on settings where the first tab is the hardware acceleration tab where you can disable hardware acceleration by un-checking the checkbox for Enable hardware acceleration and clicking close in the settings window. Disabling hardware acceleration may make flash videos and games run better on slower, and older computers with slower video cards. But, if you have a newer computer, you should keep hardware acceleration checked in Flash settings for best performance when using flash to play games, or watch video.