After long use Firefox starts to lag like crazy with Flash. And... It doesn't seem to be a problem with Flash. It's more likely that the browser itself what is causing the issue.
After I use Firefox for some time Flash starts lagging badly. BUT! This happens only if I have my Firefox window active. If I open the Task Manager on top of Firefox window, Flash stops lagging as if Harry Porter himself cast a spell with his magic stick making everything work like a charm.