This occur when there is conflict in players in firefox for ubuntu.
To remove the conflict install the Flash-Aid plugin for firefox. Get it Here
After installing Flash-Aid run it for the first time, in wizard mode.
This addon will list all the flash player installed in the system. If this list the gnash as installed, then you’ll need to remove it from the system. To do it open the Synaptic Packet Manager.
From the synaptic packet manager uninstall the gnash. And then restart the firefox. Bingo !!! The videos will be played in firefox.
If this doesn’t help you, there might be other flash tools that might be conflicting with each other.