Clear the Flash cookies and settings.

Flash Website Storage Settings panel:
http://www.macromedia.com/support/documentation/en/flashplayer/help/settings_manager07.html

See also Flash Local storage settings:
http://www.macromedia.com/support/documentation/en/flashplayer/help/help02.html

See What to do if Firefox won't play any sounds and Fix common audio and video issues

After Firefox updated to 3.6.12, my homepage stated that I did not have the Flash Player installed. The latest player was installed, but I uninstalled and attempted to re-install. The installation process worked, but the Adobe Flash banner on their page does not Flash like when a proper install occurs. The Adobe DLM add on will not allow me to change options on Firefox Addon page. Now Flash content on homepage will not play, and I am forced to use IE to view content.

My problem with 3.6.12 is that it is crashing whenever I try to open Word documents on certain webpages. Never happened with earlier versions. I have been on Live Chat but had to bail when it took over an hour. I also found out how to get a crash report so have that info and will get back on Live Chat when they are "open" again. So frustrating!