Because the use of the default
flashplugin-installer v.11.2 entails this problem, and in a more severe form, as the answer accepted there doesn't work, I have preferred, instead of digging farther in that direction, to use this solution to install Chrome's pepperflash plugin in Firefox.
That works, excepting the full-screen... The screen is black or in some cases there is a ridiculously tiny remnant of the video in the top left corner :)
There is a configuration file that can be downloaded from here and should be renamed to
freshwrapper.conf and put in
But I don't know which parameters to change there.
(I'm in eOS Freya - based on *ubuntu 14.04)