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