A memory leak implies that memory is not returned to the system after shutting down Firefox. This might be memory gluttony and not a leak. (In my opinion, memory is there to be used, and it's only a problem if other applications have to swap to disk. But I digress...) You can easily disable plugins on the following tab. However, using the web without Flash is more and more difficult: orange Firefox button (or Tools menu) > Add-ons > Plugins category