1) In Firefox go to Tools > Add-Ons > Plugins. Is the "Shockwave Flash" plugin installed and not marked as disabled? Is it 11.2 r202, the current version?
2) The easiest way to install Flash is to use the partner repository. Try this:
a) Use "sudo nano /etc/apt/sources.list" to edit the list of repositories. Find this line:
and remove the hash mark ("#") at the beginning of the line if it is there. Type Ctrl-X, then say Y when prompted to save the file. (If you are using an older version of Ubuntu, "precise" will be replaced with something like "oneiric" or "lucid".
deb http://archive.canonical.com/ubuntu precise partner
b) Now run
sudo apt-get update
sudo apt-get install adobe-flashplugin