This has been a problem since the late 3.6.x versions of Firefox and it seems to be affecting the new 4.0 version as well. It seems the browser will not release memory allocation for Flash-based web applications until the web browser is closed. You can navigate away from the site/application, however, the memory consumption for the firefox.exe process will not release. I've had the browser running at well over 700,000k, making it almost unresponsive to any input or navigation attempts. The best way to test this are with Facebook social games, such as Mobsters 2. These games are all browser Flash-based games.