Firstly, you have to check if your Firefox supports Flash player. You can do that by opening directly any page including a Flash like Adobe site to know if it works well. Another way is that you can check what installed plugins in your Firefox. I have run these commands in Ubuntu Linux for checking Flash plugin.

1. Test a page with a Flash inside


2. Check installed plugins
firefox about:plugins

If the Flash plugin is not yet installed on your system, you can use this code to install it easily.
sudo aptitude update
sudo aptitude install flashplugin-nonfree

Now you can try again and see what is result in your Firefox.
If the problem isn't fixed yet, please continue to read this post.