another thing you can try to do because maybe the shockwave is not letting the libflashplayer.so plugin work (it happened to me)why don't you try to disable or even try to move the shockwave plugin to another place (that's what I did) example your /home folder so you don't lose it and leave the libflashplayer.so in /usr/lib/mozilla/plugins, restart the firefox (close it and open it again)and see if it works (it worked for me, well it was not mageia but it is on mandriva) but I think it should work, if it doesn't you still have the shockwave file in your /home folder to move it back to /usr/lib/mozilla/plugins. but you need to be certain which one is the shockwave plugin in your /usr/lib/mozilla/plugins so you don't move another plugin.
I did that with mandriva to resolve the conflicts that I had, once I did that it let me watch youtube or any other site that requires flash...