How are you installing flash? The flash plugin from the Ubuntu repos should work fine. If you are using the flash plugin from adobe then remove that and install the flash from the Ubuntu repos.
If that don't work then create a new profile. Move your ~/.mozilla folder to ~/.mozilla.bak and relaunch firefox.
If that don't work then uninstall the flash plugin from the Ubuntu repos. Then download the linux .tar.gz flash installer from adobe.com:
Then untar it (use the exact name of the file here):
tar -xzf install_flash_player_10_linux.tar.gz
Then copy the libflashplayer.so from the directory that is created to /usr/lib/mozilla/plugins:
sudo cp /path/to/libflashplayer.so /usr/lib/mozilla/plugins
The restart firefox.