Background

The HTML 5 video player is the default video player for sites like YouTube, I guess it's supposed to be a platform independent replacement for Flash video (FLV) , but it still has a few issues. It works fine on my desktop, but my notebook is pretty old and cursed with the thoroughly awful Intel 915 graphics chip, so video playback is a little rough. Adobe Flash, for as terrible a piece of software as it is, does slightly better on this machine, especially in fullscreen mode.

Procedure

The HTML 5 player is enabled by default and there's no immediately obvious way to un-enable it, but it can be done. Open a new tab and go to about:config, click through the warning and you should see a list of variables you can set. To watch YouTube videos in flash you'll need to change two variables. Look for media.fragmented-mp4.exposed and media.webm.enabled and set both to false by double clicking on them, and well that's pretty much it. I've tried this on Firefox 42.0, which is the current version as of this writing (11/2015)  on Windows 7 Professional and it works perfectly. If you have any questions or you find that the procedure for this changes in the future, let me know.