Visit the flash player about page,
Adobe - Flash Player
What flash player does it say you have installed?
You will have to install the flash player in IE and again in firefox as they use a different plugin system.
Do you have any plugins installed into firefox that could be blocking the content?
If the above site says you have the flash player installed and you still have the issue, see if you can see the following example
http://koali.com.au/projects/webdu/censor/censor.swf
It is just a swf file so will run without any embed or detection code. If the example works then you have JS turned off either via a plugin or in your settings.
If flash works and JS doesn't and you you can't find where it would be turned off, post again and I'll continue the thought process.