According to Inside Firefox - The Inside Track on Firefox Development:
For those who remain concerned, here's how the feature works. Firefox has a preference
browser.sessionhistory.max_total_viewerswhich by default is set to -1. When set to this value, Firefox calculates the amount of memory in the system, according to this breakdown:
RAM Number of Cached Pages 32MB 0 64MB 1 128MB 2 256MB 3 512MB 5 1GB 8 2GB 8 4GB 8
Bottom line: setting
browser.sessionhistory.max_total_viewers to something low will help avoid this memory hogging.