Can't play any Flash videos

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http://askubuntu.com – Recently installed Lubuntu 12.04 with alternate installer on Compaq Evo N400c (850Mhz PIII, 20GB ATA HDD, 512MB RAM). After installation of restricted extras and medibuntu codecs I realized that it can't play videos. I tried Midori, Firefox, Chrome, Chromium but all the browsers say: "Shockwave plugin cannot opened...." I tried Adobe plugin, no succes. I tried Gnash, a little success because it plays the videos but it is too slow, cannot enjoy the videos! I downloaded the FLV file but I could not play it with VLC/gnome player etc. Any Idea? (HowTos)