Re: Ohh, Gee...
Memory use in browsers is driven by the size of the DOM and cached resources. DOMs for modern websites, especially those with lots of features (web-mail pages and anything that looks like an application) can be astonishingly high.
The Chromium browsers avoid some of the problems by creating a new process for each tab (uses even more memory but reduces the chance of one rogue tab bringing down the whole browser). Firefox is supposed to be moving towards a similar model.
FWIW I don't use Firefox as my main browser, I'm certainly not a fan of either the "sharing" shit nor out of band feature releases.