Thanks to all your great advices, I have found the (for me) best solution at
the moment:
1. Lightspark installed
2. Linterna Magica installed (unfortunately, with linterna magica, there
appears this bug from time to time:
https://trisquel.info/en/forum/en-fin-dadresse-url# so I have to turn it on
and off)
3. VideoTube installed (like linterna magica, also a greasemonkey script)
golem.de runs with lightspark (very buggy but it seems to work sufficiently
good), the other site (4players.de) runs with linterna magica and the
non-html5 youtube videos also work with linterna magica.
To sum it up, linterna magica filled the gap!
I have to test this for a while, perhaps some bugs appear.
