Try a safari extension such as Fastest tube or Youtube center. I use youtube center on mavericks and it also blocks out those annoying youtube ads.
I've uploaded the safari extension to my dropbox. Download it by clicking on the link below. When downloaded, click on it to open it, open safari preferences and click install. Once installed check that it's enabled before quitting safari and reopening to use the extension. When you watch a youtube video you'll see a download button near the bottom of the video, click on that and you'll be able to choose the both the quality and format you want to download the video in.
https://dl.dropboxusercontent.com/u/41388906/YouTubeCenter.safariextz
How do you remove these safari extensions when at some point go time you feel you no longer wanna use it.