You can try starting in safe mode, just in case one of your add ons is messing it up. If this doesn't work, you can try getting an add on that will force youtube and vimeo to use flash instead of html5.

Start Firefox in Safe Mode {web Link} by holding down the <Shift (Mac Options) > key, and then starting Firefox. Is the problem still there?

In the address bar, type about:crashes<enter>. Note: If any reports do not have BP in front of the numbers, click it and select Submit.

Using your mouse, mark the most resent 7 - 10 crash reports, and copy them. Now go to the reply box below and paste them in.

The reason it kept stopping y tube was in search macafy was checked when I unchecked everything is ok thanks