Luckily, some developers think about it too and have decided to create an add on for Firefox that adds the desired button when we watch a video in Youtube using Firefox.
Not only does it include the download button, but it includes many buttons to offer different download formats: mpg, mp4, avi, flv, HD,...
Yes you can decide the quality of the downloaded video, now that HD is trendy, you can download high quality videos from Youtube to enjoy them offline.
You don't have to do extra actions, just access youtube and you'll view the buttons just down the video, choose the output format and click the button. You'll have the video downloaded within some seconds.
Don't hesitate. This is probably the easiest way to download videos from youtube.
It requires Firefox.