What version of flash player do you have? Do you have the non-free version?
If you don't know or do not have the non-free version install it. It works way better than the default version that is installed by ubuntu and Debian.
To install it open a terminal and type this command
sudo apt-get install flashplayer-nonfree
If you do not have it installed it will install it, if you do have it installed it will say it is installed and the newest version
After you install flashplayer-nonfree, restart firefox.