There are many ways to get flash working on firefox, but here is a quick, easy, clean way to do it.
1) Go to http://www.adobe.com/go/getflashplayer
2) Download the tar.gz tarball
3) tar -zxf Flash_Player_10_forLinux.tar.gz
4) cd Flash_Player_10_for_Linux
5) Run the installer, when asked for the location of firefox, enter /usr/lib/mozilla…
If for some reasons you still experience problems… then manually copy the libflashplayer.so file which should be at the root of your untarred flash tarball to /usr/lib/mozilla/plugins/