This is most likely from the webpages them selves. If you have standard webpages up like Google,Yahoo,Facebook. You should not be anywhere near that, It should be around 300,000k or 300MB.

But if you have 10 webpages of high usage pages like YouTube,ruinescape or other online games This would be your problem.

Most of the pages are from gizmodo/lifehacker/engadget articles to read later, that is all I'm doing with my FF. I did have one youtube clip loaded when I measured the usage but it was only 1minute long.

Does that also happen if you don't have any tabs open?

You can also check memory usage on the about:memory page via the location bar.