[sohkamyung]

In Firefox, entering about:performance in the URL bar will give you some statistics on how much memory or CPU each tab is using.

Entering about:about in the URL bar gives you a short list of interesting about:* links that you can play around.

[Zardoz84]

I just saw that I'm using more RAM that I thought, but Tab suspender really works fine to keep ram usage at bay.