I had also problems to play Flash on my Ice Cream Sandwich 4.0 tablet, it seems it's not compatible. As far as I tried until I accidentaly broke my tablet, there's no way to fix that and the issue is located on Flash, not on Firefox, because I tried also in Android's built-in browser and didn't work. Nor it worked on any app that used Flash. We're sorry for the inconvenience. If you were trying to play videos on Youtube, you can easily play them by visiting http://m.youtube.com/, which will bring up native video player instead of Flash when playing videos. Also some other video sites have a mobile version featuring rtsp instead of Flash. Check it out.