I see I am late...but I'll comment anyway.

With the recent ability of browsers to have multiple tabs open and save sessions, you may open to one tab and forget, as WYS did, that 'stuff' may be going on in other tabs...depending on what you left open last time. I have had a browser seem slow, then heard noise, and realized that another tab had some YouTube video running.

My solution, such as it is, is to put regularly watched but heavy-on-the-resources sites on my 'personal toolbar', or in bookmarks, and not leave them open.
(and it's different in Opera as opposed to Firefox )