Open your Firefox and go to here to install the plugin. Once the plugin is installed, you can click on the icon on the right top hand corner to install the correct Flash.
The official wording of Flash-Aid :
Remove conflicting flash plugins from Ubuntu/Debian Linux systems, install the appropriate version according to system architecture and apply some tweaks to improve performance and fix common issues.
That's all! See you.
UPDATED on May 23, 2012 :
If your problem is still there and you have nVidia display card with "
libvdpau1" installed, you should follow the steps below to solve the problem.
sudo add-apt-repository ppa:tikhonov/misc
sudo apt-get update
sudo apt-get install libvdpau1
This solution is workable on Ubuntu 12.04 LTS with flashplugin-installer 126.96.36.199ubuntu0.12.04.1 but not with Flash-Aid 2.2.3.