In FF6 (available right now on the Aurora channel and in the next few days on the Beta channel) and later versions you can go to about:memory and click on the "minimize memory usage" button at the bottom. Maybe do it a couple of times for luck.

We're working on improving things. See https://bugzilla.mozilla.org/show_bug.cgi?id=656120 for another big improvement that will go into FF7.