Firefox 7.0.1 does not recognize that Adobe Flash 11 is installed. it works with Safari, not Firefox
Have you looked at the Knowledge Base article Flash Plugin - Keep it up to date and troubleshoot problems? There is a lot of information in there on updating and installing the Flash plugin.
Hopefully this helps!
You have to run Firefox in 32-bit mode in order for it to run Adobe Flash Player 11 on Firefox 7.0.1 on Snow Leopard.
Mac: Get Info (Command+I) on Firefox icon and click box run in 32-bit mode. I think its shoddy Mozzilla support and really lame that they didn't post this.