With the newer versions of pretty much all browsers you'll get that message - esp if your system has enough free RAM -- the browsers are caching more and more info. since most systems now have more RAM and can afford to use some of that memory and they are designed to release some of that cache if it is needed by the system for something else so I wouldn't worry about that message esp. if you've been viewing several videos or have lots of tabs open. (It's just a warning message the AV products include to let you know the browser is using quite a bit of memory and their threshold doesn't take into account the higher resolution of youtube videos etc. that are being used nowadays so tend to get displayed much more often) -- The only time I'd worry is if it starts to slow performance in which case simply closing and reopening the browser will free up much of the memory since the cache will be cleared.
I use Firefox and AVG and get the same thing -- if you really tire of the notification popping up you can disable the warning (at least in AVG and I'm sure Norton should have the same option) -- for AVG it is under advanced AVG settings -Display AVG advisor performance notifications -- Screenie of setting page ( http://techdows.com/wp-content/uploads/2011/09/Disable-... ) Unfortunately there is no way currently to increase the threshold of the warning but hopefully an update will either increase it or allow users to do this !! -- It was implemented to warn of memory leaks within the browser but as the resolution of videos and the cache sizes have increased more and more it is just an annoying message since the usage that it warns at is much too low for current systems.