billege- try disabling 'fastback', which stores a number of older pages in memory for 'lightening fast back and forward button use'. The technical information:
Using Firefox 1.5 caching.
To disable: in about
:config, create a new integer preference 'browser.sessionhistory.max_viewers' and set it to 0. I think it requres a restart as well. (There's already browser.sessionhistory.max_entries and browser.sessionhistory.max_total_viewers; ignore them.)