["solved"]
Thanks for helping.
Flash has got a breakage soon after the installation,at first try,with a youtube(among the more compatible format)video.
I didn't have runned apt-get for updating / upgrading still as very often brings problems in Ubuntu.My pc is old and some upgrade in the kernel after 10/10 ( better, with 2.6 image)completely breaks the system,causes kernel panic.So upgrades to testing in deb.
About flash,it's not first time I have some matter.
I removed the default app.,than trying also other solutions,not togheter,however.I also tried gnash -the oldest- which is still default in debian.But without reinstalling firefox,I didn't thought to.I think somewere in some config file must place the problem.Not a great matter,as
I'm going to reinstall.This distro it's not for production,just to learn now.I will report about it.