Flash Fix @ Ubuntu ...

My flash If you want to completely remove all flash plugins(& install again later), run the following command and then delete the related plugins inside ~/.mozilla/plugins, if there are any:

Code:

sudo apt-get remove swfdec-mozilla
sudo apt-get remove mozilla-plugin-gnash
sudo apt-get remove adobe-flashplugin
sudo apt-get remove flashplugin-nonfree

Keep in mind this will also remove flash plugin from Opera, since it's not Firefox that installed the flash plugin. The flash plugin installation is generic for all browsers.

Nevertheless, I believe your problems is due to conflicting plugins. So before trying to remove all flash plugins, try this(IT WORKED FOR ME):

Code:

sudo apt-get remove swfdec-mozilla
sudo apt-get remove mozilla-plugin-gnash
sudo apt-get remove adobe-flashplugin
sudo apt-get remove flashplugin-nonfree
sudo apt-get install flashplugin-nonfree

From my experience, flash does not play well on any browser. Nevertheless, you might experience better performance with Firefox 3.5.3, which is faster than Opera. The alpha releases of Chromium and Firefox 3.6.1a are the best for flash.

You can also try to improve your flash experience using my tutorial Firefox optimization and troubleshooting thread.
Source : Ubuntu Forums