You may be wondering why despite your preference settings Firefox 3.5 still keeps in memory the browsing history pages visited way back in the past. This despite settings in Preferences -> Privacy menu. Actually that setting is named 
Remember my browsing history for at least n days
because it really sets the minimum time the browsing history is kept in memory. In order to set the maximum time you have to change a key after you type in the URL bar 
about:config
Using the search bar provided look for this key: 
browser.history_expire_days
Default: 180 days You can also set the maximum number of webpages to be recorded into the history by setting the string 
browser.history_expire_sites
Default: 40000 pages