If you start using Firefox 3 with Flash, odds are good that you'll experience Firefox crashing every few minutes while loading Flash. It seems Pulseaudio (Hardy's new sound system) is triggering a bug in Flashplayer, which causes Firefox to crash. Since Flash is closed-source software, there's no chance Ubuntu is able to fix the problem in time for Hardy Heron's release.
First you need to download and Install nspluginwrapper from here
Now you need to install libflashsupport using the following command
sudo apt-get install libflashsupport
Now Run the following commands
sudo apt-get remove
sudo apt-get install flashplugin-nonfree
Now you need to Restart firefox