current versions should install as Adobe Flash not Shockwave Flash
Best bet is to uninstall all versions of both
Then run Flash uninstaller from
Adobe - Web Players to make sure everything is gone
Then reinstall Flash from within Firefox (to make sure you get the right version)
If that still fails post back - theres a Flash diagnostic cleanup tool which can be used as a last resort, but I need to look it up ---its been a long long time since I needed it