I'm getting the same issue, except no sound (for youtube videos).
I tried removing flash in Software Manager, and re-installing but that didn't work
I tried removing flash and then re-installing firefox, then instal flash
I went to my mozilla /profile folder and deleted everything, re-installed flash, but still no luck
I'm new to Linux so I'm not sure if that was a good way to go about this issue.
I realize this thread is 2 days old without a reply, so I assume I'm lost for this cause. Tried Ubuntu for a week before switching over to Mint (MATE).. grrr, I'll proabably be heading back to Windows