I have been plagued by an issue in Firefox when using Flash on 64-bits of Ubuntu, from around version 10.10 This issue was whenever I visited a website that used Flash, the Flash content would flicker, with white spots all over the Flash content. This was very annoying as I was not able to access sites such as Youtube. To get around this I just used Chromium for sites that used Flash.
However recently I discovered this was an issue with version 10.1 of Adobe Flash player, and that using the 10.3 beta solved the issue.
Here is how to install it. In the terminal window type:
sudo add-apt-repository ppa:sevenmachines/flash sudo apt-get update sudo apt-get install flashplugin64-installer