Firefox 19 Video playback problem.

Hi Guys

Currently have Firefox 19 installed on my 2011 27" iMac with Mountain Lion.

Everytime i stream video's from Youtube then scroll down to to comments or change tab, then go back to the video, the video becomes choppy, but sound it fine?

Anyway to fix this?

Currently using Chrome which im starting to like, but would like Firefox to work correctly if possible.