How come web browsers are so heavy in memory? I have Firefox opened with two tabs, and it takes over 800 MB of RAM, Chrome with four tabs takes over 1.2 GB! I mean, what's up with consuming so much memory?
From my experience, Chrome is a bit free on the system resources, especially if you go to some heavy sites (if it can use more memory to make your navigation faster, it will).
About Firefox, the weight is usually caused by the number of plugins and toolbars you have installed. Try intalling it again in a separate place, and install only the minimum of plugins.
Opera is not the lightest, but it has reasonable memory use in my opinion (amongst other great functionalities in my opinion, but this is not the place). Also, you can't install plugins besides the included ones, it helps with not cluttering it.
For information, here is a recent (as of today, at least) comparative between browsers, to give an idea of the speed and memory uses you should expect.
Minimizing tabs you are not using is made easy by the free OneTab extension
Use "The Great Suspender" Chrome extension. It greatly reduces memory used by chrome(and other chromium-based browsers. Link: https://chrome.google.com/webstore/detail/the-great-suspender/klbibkeccnjlkjkiokjodocebajanakg