I had this problem of flash content not loading properly in Firefox on Ubuntu. That means I can not view any YouTube videos.
The issue was the default plugin for flash in Firefox was gnash. It seems the gnash is still not there or the sites are spoofing flash player version and not showing us the content.
Anyway to fix this I have to remove gnash.
sudo apt-get remove mozilla-plugin-gnash
Then I have to install
sudo apt-get install libflash-mozplugin
That started playing flash but sound was not there. So I added
sudo apt-get install libflashsupport
And that’s it. Everything started working as expected.
I have used apt on command line as I am used to CLI. You can do it using GUI package tools as well.
So now I am enjoying YouTube videos on Ubuntu and Firefox.