Firefox only: The previously mentioned BarTab extension lightens Firefox's memory load and prevents crashes—in other words, it's a great add-on. But the latest Firefox 4 betas, and presumably the final release, can do most of that with an about:config switch.
CNET's Seth Rosenblatt details in a nice video package the whys and hows of this Firefox configuration tweak , one that should work in the most recent Firefox 4 beta:
If you're an experienced Firefox-er, head to your about:config section in the latest beta, search out browser.sessionstore.max_concurrent_tabs, and set it to 0. You might still appreciate the BarTab extension for the extra options and control it provides, but if you just want Firefox to give you back your memory when you're not actively looking at a web page, there you have it.
Speed up Firefox 4 with a BarTab tweak [The Download Blog/CNET]