Not a fix but you might try avoiding large extensions that offer very little benefit. Also see if you have extensions mentioned in "[http://kb.mozillazine.org/Problematic_extensions Problematic extensions]" and if you do read about problems that you may encounter and may have to make some adjustments for. I would suggest installing the "[https://addons.mozilla.org/firefox/addon/restartless-restart/ Restartless Restart]" extension (12KB and is for Firefox 4,0 and above) and perhaps invoking it when you leave the room so Firefox will be restarted where you left off when you return. Be sure to reply to any messages about closing tabs before leaving. Since you are 3.5.6 instead install the "[https://addons.mozilla.org/firefox/addon/quickrestart/ QuickRestart]" (35KB) extension instead. You are on Windows 7 you can use the latest Firefox version 8.0, and 3.6.* would be better than what you have. Some of your problems might be lessened.

Are you using AVG?

AVG is reporting high memory usage levels not CPU usage on on the popup task bar messages that I see.