Hey guys, I had this problem, and i was able to fix it in my case.
So on 64 bit ubuntu, the flash plugin has issues, mainly that it doesn't really work all the time, because there is only the x86 (32 bit) plugin, and no 64bit version. I realized this was the issue with the web browsers after upgrading to intrepid because it was flash-heavy sites (cnn.com) that would cause it to hang and go grey, but not simpler sites (digg.com). Also, running 'gksudo firefox' will give you the errors regarding not finding the flash libraries when you go to those flash-heavy sites. I had fixed flash before the upgrade thanks to a script, but the upgrade screwed things up.
So, if this is a similar case for you, run this script below, which will download all necessary libraries and the Flash 10 plugin/player and *wrap it*, so it will work correctly with the 64bit system. After restarting firefox, enter 'about:plugins' in Firefox to check if shockwave flash plugin is installed, it might be at the bottom.
# Script created by
# Romeo-Adrian Cioaba email@example.com
# Super minor updates by jason.melton[at]gmail[dot]com
# Other minor updates by firstname.lastname@example.org
# Released under GPL
#Must install Debian package 'getlibs-all' first:
#This is a package that will grab libs, 32 and 64-bit under amd64 (and 32-bit only under i386.)
echo "Getting and installing getlibs-all"
sudo gdebi getlibs-all.deb
rm -rf getlibs-all.deb
echo "Stopping any Firefox that might be running"
sudo killall -9 firefox
echo "Removing any other flash plugin previously installed:"
sudo apt-get remove -y --purge flashplugin-nonfree gnash gnash-common mozilla-plugin-gnash swfdec-mozilla libflash\
sudo rm -f /usr/lib/mozilla/plugins/*flash*
sudo rm -f ~/.mozilla/plugins/*flash*
sudo rm -f /usr/lib/firefox/plugins/*flash*
sudo rm -f /usr/lib/firefox-addons/plugins/*flash*
sudo rm -rfd /usr/lib/nspluginwrapper
echo "Installing ia32-libs and nspluginwrapper"
sudo apt-get install ia32-libs nspluginwrapper
echo "Getting libs"
sudo getlibs -p libcurl3
sudo getlibs -p libnss3-1d
sudo getlibs -p libnspr4-0d
echo "Installing Flash Player 10"
tar zxvf flashplayer10_install_linux_091508.tar.gz
sudo cp install_flash_player_10_linux/libflashplayer.so /usr/lib/mozilla/plugins/
rm -rf ~/install_flash_player_10_linux/
sudo nspluginwrapper -i /usr/lib/mozilla/plugins/libflashplayer.so
echo "Linking the libraries so Firefox can find it."
sudo ln -sf /usr/lib/nspluginwrapper/plugins/npwrapper.libflashplayer.so /usr/lib/mozilla/plugins/
sudo ln -sf /usr/lib/nspluginwrapper/plugins/npwrapper.libflashplayer.so /usr/lib/firefox-addons/plugins/
rm -rf flashplayer10_install_linux_091508.tar
echo "Done :-)"
#Enter 'about:plugins' in Firefox to check if shockwave flash plugin is installed, it might be at the bottom.
Give that a try, it might solve your problem. If you are *not* running 64bit, try reinstalling flash either via firefox plugins or via ubuntu's add/remove applications, and restart firefox. Let us know how you fare.