Hello brenda4 you may like to try this, it works very well on my Firefox 11 beta4 XP system.
A preference that will (on tab close) if plugin loaded essentially release plugin containers thus freeing up memory, on tab close.
Type: Boolean
Preference Name: plugins.unloadASAP
Value: True
1:Type about:config in the address bar/enter
2: Right click and select New/Boolean
3: Preference Name: plugins.unloadASAP
4: Set to: True
Hope this helps. I believe there is a bug with this in Firefox 12 and above.