All major websites (such as Youtube and Facebook) are moving away from using Adobe Flash, which presents certain security risks as well as risking browser instability, in favour of using HTML5. For that reason Firefox will stop automatically playing Flash content when you click on it:
https://blog.mozilla.org/futurereleases/2016/07/20/reducing-adobe-flash-usage-in-firefox/
However you can still view the content, if you wish to do so, by simply clicking on 'allow'. (Always ensure that you're visiting a trusted website before allowing Flash content to run).