Firefox saves the last 50 pages it browsed IN MEMORY. Because, you know, FASTER to display.
Firefox has a habit of prefetching pages from links it thinks you might click. (and they pile up in that 50 page bin)
It's possible to tell Firefox to garbage-collect its memory use when you minimize the browser, which makes it MUCH faster.
These three tricks are done as follows:
1) open a new firefox tab. enter "about:config" in the address bar.
2) say "Yes, I promise to be a good little drone and not break your settings, honest."
3) find the entry browser.sessionhistory.max_entries which is set by default to 50. click on it and it will let you enter a smaller number. 50 is almost certainly too large.
4) find the entry network.prefetch.next and click on it. Set it to False.
5) right-click in the about:config window and select New > Boolean.
Type 'config.trim_on_minimize' in the box that pops up. Press Enter.
Select 'True' and then press Enter.
6) close the about:config tab and restart firefox for best effect.
My firefox image is at 198264 K memory ... I minimize it.... it drops to 167400 K ... with six large-page tabs open. Closing the tabs, it actually reduces memory usage (which I haven't always seen in the past),
Restarting the browser and comparing to a fresh, same-screenspace-sized IE on a specific webpage, IE is at 39256 K where Firefox is at 23,367, a profound improvement over its previous piggish nature.
Seems like an improvement to me. Of course, time will tell whether this is better for a browsing experience.