"Notice that most of these tips only free up some
memory at the expense of (usually) loading speed for web pages. You
won't make Firefox 10 times faster, but you will surely make it use
less RAM.
"Tweak the browser.sessionhistory.max_total_viewers variable
In Firefox, open a new tab (Ctrl+T) and type about:config in the
address bar. Click on the I'll be careful, I promise! button and
start typing the variable name
browser.sessionhistory.max_total_viewers in the Filter text
box."