And, responding to the questioned posed in the initial post...
I don't know what I was using in such experiments. But, I remembered that,
when I tried another free distro, about a year ago, I could already choose on
YouTube to use the experimental HTML5, and the quality of videos was quite
superior, compared to the ones played by Gnash. More close to the performance
of the proprietary plug-in, but still not as good...
Anyway, this is just the begginnig...
And, being a Free Software project, dedicated to something that people use a
lot, it has a lot of evolutionary potential.