There are other ways of doing this, like:
- a Greasemonkey script which replaces the Flash Player on websites with VLC or Mplayer but that was kind of buggy;
- an external YouTube player but I usually you'll find links YouTube links inside your browser so you would have to copy each link in an external player, etc. and that's too much trouble.
But you will probably find the methods in this post to be more convenient because:
- you can open any link you're browsing in Firefox, in Mplayer of VLC (method 1)
- because we are going to use youtube-dl, you can set the video quality (method 1)
- the second method works for any browser and some other websites too (not just YouTube) - I've only tried Firefox and Google Chrome but it should work with others too
- basically everything is done within the browser is it is very convenient.
- all is done with just one click (of course, setting this up takes some more steps but you must only do this once).