There are some compatibility issues between the latest Flash player and some Firefox add-ons or settings. Could you see whether anything in this article helps: Flash 11.3 doesn't load video in Firefox.

You might also consider disabling the new protected mode in Flash 11.3, since that feature is currently being debugged. See this support article from Adobe under the heading "Last Resort": Adobe Forums: How do I troubleshoot Flash Player's protected mode for Firefox?

not helpful

Hi oscarxp, which part was not helped: hanging on Flash sites, or high memory use?

well i have done what it says nothing is effective, i still get high memory when i go on site that uses flash like youtube and i view a 720p video.