Some sites like YouTube worked but others like Vimeo did not.
Use Vimeo’s Flash version checker
It seemed like the sites that weren’t working need Flash 10.
A package called swfdec-mozilla was providing Flash 9 to Firefox even when
the flashplugin-installer package was installed to provide Flash 10.
sudo apt-get remove swfdec-mozilla
sudo apt-get install flashplugin-installer