System slowing down and high memory usage

My problem is about memory usage when viewing flash content in Firefox. With one Youtube video running, Working Set Memory for the Flashplayer plugin in Windows Task Manager is pretty stable at around 100K.

PHYSICAL memory, however, which is given at the bottom of the Task Manager window, steadily increases, and after watching videos for two hours has risen to 70%-80%, causing Windows to slow down greatly.

After shutting down the plugin in Task manager ( which often takes a minute or more) physical memory goes back down to around 17% and the computer becomes usable again.