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
Using the search bar provided look for this key:
Default: 180 days
You can also set the maximum number of webpages to be recorded into the history by setting the string
Default: 40000 pages