I have most of the multi media ones enabled, along with a Logitech Harmony and Citrix plugins. As for exentions I have have Firebug, Web Developer, SQLite Manager, Memory Restart and BetterPrivacy. It may be time to reevaluate which plugins and extension should be running.

Other people mentioned the about:memory tab. Taking a look at it now, I'm seeing about 80-100MB difference between what that tab say and what the activity monitor says.