I had this exact same problem and found a fix for it.
1. Go to your Add-ons Manager.
2. Click on Plugins.
3. You will see "Shockwave Flash" with a blue link under it that says "More". Click on that.
4. You should see a screen that says "Shockwave Flash 23.0.0.185. At the bottom it will say "Enable Shockwave Flash protected mode" with a check mark.
5. Uncheck that, and re-start Firefox.
This should fix the problem, well at least it did for me.