Which version of Debian?
Firefox or Iceweasel? And which version?
apt-cache policy iceweasel
What version of flash? Adobe Flash or Gnash?
apt-cache policy flashplugin-nonfree
If you are using Adobe flash how did you install it? Using flashplugin-nonfree from the Debian repos or flashplayer-mozilla from the debian-multimedia.org repo?
The more information you provide the easier it is for people to help you.
Go into the Firefox menu and look under Applications and make sure Gnash is not being used.